This means the card is so large that it takes the space of two slots in your case (and perhaps covers a second slot on your motherboard).

It more like 20-25% faster, not twice as fast.

Just beware of the micro-stutters that go along with SLI (or crossfire, either one).
If you feel those might bother you, you may want to get a single core card.
It was said sometime back that both nVidia and ATI are working to eliminate the micro stutters. I don't know if ATI's new offering has taken care of this or not.
Currently any nVidia card, be it native dual core or two cards runs in SLI will have micro stutters. Hopefully the G300 will fix that.

Micro stutters? Awe crap! First time hearing about those.

You can find all kinds of information on it. Just google sli micro stutters.
Both NV and ATI stated that nothing can be done without a change in the cards architecture.
I waited for the GTX 285 to come out before I bought my card for this build because of it.

Most people say they get used to it. Some say they don't even notice it. Some couldn't stand it and went to 1 single core card. I have seen it and I knew I wouldn't like that happening, so I decided then to not go SLI this time around.

But, chances are both NV and ATI will have this corrected with the new cards. ATI may have done it with the 58xx, I don't know. NV should have it worked out with the G300, whenever that may come.

I have never had a micro stutter, and chose the 295 because I did not want to have them. No micro stutters on a dual-gpu card, at least, not this one.

I had the 285 at first, but was not satisfied with it, and my 295 is 74% faster in almost every game/sim. To me, that is almost twice as fast.

-Skater

...But, don't take my word for it...

http://www.anandtech.com/video/showdoc.aspx?i=3501

-Skater

Like I said, do a search for it. Plenty of information on it. Statements from both nV and ATI about the problem, what causes it and how it can't be fixed with the current architecture. Both have said they would work it out in their next gen cards. ATI's new one might very well be micro stutter free, I don't know. But any nV dual core or two cards in sli do have it happen. It's a matter of the technology used.
From what I understand about it, it isn't the drivers, the games or settings, it is caused by the way the two cores communicate between each other and is indeed inherent in any sli setup, be that a dual core single card doing sli via that one card, or two cards in sli. It doesn't matter, they will have micro stutters.
Anyone thinking of going sli should read up on it before you decide what to buy. I am glad I did, it kept me from making a big mistake.
And also like I already said, most people either;
Say they have gotten used to it.
Say they don't notice it much.
Say they can't stand it.

So it is safe for me to say;
best and fastest Nvidea Card available?
Best, GTX 285
Fastest, GTX 295

To finish what Tailflamer said:

Best (most problem free): GTX285
Fastest (currently on the market): GTX295

But actually available:

GTX275 and lower.

Maybe availability for the 295 is better since it's damn expensive, needs more power than the 285 and runs hotter - it's an extreme card for extreme people IMHO. Finding a GTX285 is seriously hard these days.

My conclusion against the 295 was made because often when I see new games released, the bug report forums are full of "My dual-core card or SLI setup performs much worse than expected".