Ok, I know what SLI is, running 2 Nvidia cards in the same PC, but not sure what benefits it actually gives for playing games. I have often heard people saying it's a waste of money, that it doesn't make anything faster but then why do people do it?

I have also heard some people having games run worse because of SLI (and the ATI equivalent) but could be other factors involved there.

Why I'm asking is that I can get another 660 (the exact same one as I have in now) for a just over £100 so if there are larger benefits than buying a new card I would consider it. Also, is there a minimum PSU requirement for SLI?

2 cards, twice the power required to run them. I think mine is 900 watts, a good power supply and does fine. Probably a bit more than needed, but better to have too much than too little power.
My cards are crossfired, a 6950 and 6970. I get a few more fps in most games, but some games just don't make use of it and it makes no difference or I might get a couple extra frames by disabling it for that game's profile. It's not the game changer I had hoped it would be.
I got the second card in hopes of spending a little to get more life out of the first card. Would've been better to buy a new card a couple of steps up and sell the old card.
I won't bother with it again.

Well if the games you play actually benefit from SLI then it is a good Idea.. There are benchmarks out there for single vs SLI setups and I have seen increases in a lot of newer games..

Or if you game on more then a single monitor SLI/Crossfire is a benefit..

I would run a PSU Configurator to see what kind os PS
U you will need wattage wise but remember the amps on the 12v rail are just as important.. so see what kind of wattage you need and the Amp draw on the card..

I was in your position a couple of years ago when the 460's went down to about £100. The difference was fantastic for the price, I get performance roughly equivalent to a 580, which at the time was £4-500 so it really was quite a bargain. The only downside is the odd game that doesn't support it, Cliffs of Dover is a notable example for myself, and is the only game that I actually have to turn SLI off for. BF3 BF4 and most modern games have near 100% scaling. Oh yea Rome TW2 is another notable one, but all the other TW games scale very well. I expect Rome II to catch up once the many issues are sorted out. PSU wise mine is a 750 Watt Antec Truepower new which copes with two OC'd 460s and a 2500k @4.2 Ghz quite nicely.
I'd double check that your Mobo supports SLI (not all do) and that none of the games that are important to you aren't supported.
Oh and I would recommend using MSI Afterburner or EVGA precision to make sure both cards are clocked identically. My cards were Oc'd out of the box to different specs so I made sure to match up both the memory and core clocks.

SLI (and CrossFireX) make sense to me only as a cheap upgrade (such as you may be able to do). That is, when one has an older card and can get a second one very cheap.

I've CrossFired three times along the way -- 2x HD5830, 2x HD5870, 2xHD7970 (current set up). Mostly, CrossFire has not been a "problem". The latest AMD drivers turn it off on games where it might be a problem. The FPS improvement has be anywhere from nearly 0 to nearly twice as fast (depending on the game).

Thing is both Nvidia and AMD use a rendering method that raises frame rate, but does not improve "response time". That is, some games are "twitch" games -- and more FPS should allow faster human response to events on the screen. With SLI and CrossFireX, that does not happen.

So, SLI and CrossFireX merely smooth out the FPS in games where they work (lots of games). And, can be a cheap upgrade.

Still, the best solution for gaming is buying a single card -- fastest possible single-GPU card -- not always affordable.

SLI always seem to get a bad rap from uneducated people or people who simply can't afford that kind of setup, then rely heavily on what the internet says. I can tell you from experience, going through multiple SLI setups that it's downright amazing.

2 cards have a scaling of almost 100%, meaning you will get double the framerate. 3 cards it drops to around 75% and at a quad setup it's just a waste of money.

There's two issues you will see brought up constantly; frame-times and "stutter". Yes, frame-times will increase from an average of 8mss to 16ms by having an SLI setup but I can assure you as a competitive gamer and owner of a 115Hz monitor, you simply won't notice.

Secondly people claim "Oh it adds stutter". No, it's simply not true. That is most likely on that person's end because through 4 SLI setups now and the experience is butter smooth. Yes, few years ago you could see a bit of stutter when the technology was in its infancy but most of the kinks have been worked out.

The biggest 'issue' with SLI is profiles. Bright side is Nvidia are extremely active when it comes to adding profiles. If there's a high-profile game releasing, there will be a profile in place well before release. Lots of smaller titles also get SLI support, plus many games work with cross-SLI profiles like the unreal engine.

However after having a talk with a community rep that's also on Geforce.com, in 9/10 cases when there's no SLI profile it means the developers have to do something to the engine. Developers also have to proactively build their engines for multi-GPU use. Lately I recall Ghosts, Rome 2 and CoH 2 as high-profile games without an SLI profile due to a lack of 'developer interest' or what you'd call it.

However a majority of the games I play have an SLI profile.

Also, many people seem to put SLI and Crossfire in the same bag when trying to discuss pros and cons. On that note I'll say that Crossfire is still behind SLI. Ever since the introduction of Nvidia's SLI they have been using an on-board solution to create a smooth experience, order the frames and cut down on frametimes. AMD on the other hand have had MASSIVE issues with stutter and bad frametimes, because they had no frame pacing technology. It's getting better but still not perfect.

It's only recently they started really focusing on it frame pacing, they released an alpha driver I believe not long ago that shows promising results but it's still WIP.

So for SLI specifically, you don't have to worry.

Good info from all here, I admit my not-so-great experience I base my opinions on was Crossfire (2x5870s) so maybe SLI and Nvidia drivers and profiles may be a better experience in more titles than what I saw which was not worth it at all.

Good to know it might be better as the price point to SLI my 670/4GB will eventually be hitting the $200 price point at which I might consider SLI.

MaceUK one thing I would recommend is that you google SLI and whatever games you currently play, that should get you some good info on whether those games are supported well or not supported or have issues with SLI enabled, although you should assume happy people post less than unhappy people.

Even with my previous bad crossfire experience I would still consider it if the price for the second card is a good one, and the games I currently play support it, just don't go into it expecting 200% the performance on everything.

In the process of building a new PC for a friend, I scored a pair of GTX 760s on clearance at Micro Center. One for me (GTX 480 died, so I had to replace it with something other than an 8800 GT), one for him.

I figured I'd SLI them in the meantime since I don't have all the parts I need for his build yet, and to be honest, I don't notice any serious microstutter or latency problems. All I do notice is that framerates went from "fast" to "LUDICROUS SPEED!" in the games that have SLI profiles and aren't CPU-bottlenecked.

Keep in mind that I've never run SLI before, just single-card setups. Of course, a big reason for that was being stuck on a P35 board until very recently.

This also ties into my upgrade plan for said friend's computer; it's likely that I'll move on to a new graphics card a few generations later, at which point I'll offer my GTX 760 to him so he can run SLI and nearly double his framerates.

Is there any game in particular one of you might want me to test out with SLI while I still can?

Points to consider:

I stepped away from crossfire 2 years ago do to the chronic micro stutter problems faced by crossfire setups (well known do google search) supposedly new drivers go a long way to address this but don't eliminate it all together.

Be sure to understand how many PCI express lanes you have and what data rate they are available at in X-fire or SLI. Many boards only support one card at 16x and will only support 2 at 8x and so on. So you may not reap the same benefits as a single card depending on motherboard.

Another point, every-time you update your freaking nvidia drivers it tends to default the SLI setting to: disabled. So if you update and things run like crap you'll know why. Frankly, with 2 680's sometimes I don't even notice, but with intense stuff like BF4 on ultra or 3d stuff, I do and then forget that I updated and need to turn it back on.

Also, highly recommend SLI/X-fire if you are doing any 3D vision gaming.

If you are starting a new build do your research...the cost of 2xVidcards may not be value added compared to performance and price of 1x vid card. My philosophy is to buy the best single card solution and then a year later purchase another when the price drops. But depending on your budget and build a different path may be bet for you.

It seems from years spent considering the move, to me that the "Mainstream" games mostly make good use of it, whereas niche games and simulators (Arma2 and Rfactor2 come to mind immediatly) more often don't benefit or even suffer.

In a KISS gamer build PC a dual card solution only makes sense (to me) when it is comparativley cheaper than a single really high end card. There's plenty of hassle with drivers, profiles, settings in any modern game so I always avoided that additional layer of complexity.

Also a consideration can be the additional noise and heat.

ArmA 2 had a decent SLI profile for years, many games do. Problem is a game like ArmA is massively CPU intensive so all that additional GPU power will do exactly nothing.

Also, running SLI isn't that much more noisy, sometimes even less noisy because instead of one card hitting max fan speed you have two cards running half speed because the load is shared. Only gets a bit noiser when playing a super GPU intensive games that has great SLI scaling.