The 1050 Ti is very capable, the 1060 3G more so, and 1060 6G more still. The differences are worth considering (but you have to factor in the pricing, and that can vary significantly). No doubt, if you're on a tight budget, go with the 1050Ti. If you can afford it, go the 1060 6G route. Compared to your 745, I would anticipate the 1050Ti to be about a 2-2.5x performance increase, and a 1060 roughly 3-4x, depending on which model, 3G or 6G. It is true the 6G 1060s have a few extra functioning guts and thus are slightly better than a 3G model. However, I do actually have two of the 3G cards, and can say they are more than adequate for gaming at up to 1080p resolution, depending on the game and graphics settings. I can't say about ARMA3 specifically, but both my sons played PUBG, Rainbow6, and several other FPS games using 3G 1060s and they'd run fairly well (45-60 FPS) at mostly high graphics settings.
As for the PSU, it depends.
According to online info, the Dell XPS 8700 ships with a 460W PSU - and the Dells usually come with very good quality units. But, as is always the case with PSUs, it's the load that matters. Doesn't matter how big or small (in watts) a PSU is, you have to know the load your equipment presents to the PSU in order to say whether the PSU is up to it. You should also keep in mind that power supplies, over time, actually do sort of deteriorate in such a way that they progressively are capable of less and less output that is still stable. No reason for alarm; we're not talking a 20% drop-off in two years or anything, but this is another argument for high-quality PSUs: The good units don't suffer this anywhere nearly as bad as cheap designs, because the good ones use quality components that hold up better over time.
Still, all PSUs will lose at least a little output capacity over time. In high-quality designs, this means you get about 5 years of rated output (and possibly higher), then you start to lose a little each year. At 10 years, it may be a good idea to replace the unit (though you can keep running these longer, and I have in some cases). What usually happens is they will run, but might 'act up' in warmer temps or when stressed by a certain game, etc. Even "solid-state" electronics don't last forever
It's also not a great idea to run a PSU at or near it's maximum output capacity all the time. Without going into a lot more detail about efficiency etc, let's just say that you want to shoot for 50-75% load as a broad guideline.
So, in the Dell you've listed: There are four different types of 4790 CPUs, and they all are rated differently in terms of power required, from 45W to almost twice that. Based upon your citing 3.6GHz, I'm inferring this is a 'plain' 4790 (not the T, S, or K models); it is rated at 84W. If we take the 460 PSU - a solid unit, no doubt - and figure 75-80% load (which also leaves a little overhead), you 're looking at about 350W (+/-). Subtract 84 for the CPU, and allow ~200W for the motherboard itself, RAM, etc (which is more than adequate)...you're comfortably left 75W to power a GPU. Mind you, the math we're doing now always uses maximums; in reality these components do not run at max all the time - usually much less.
The PCIexpress slot itself can provide up to 75 watts of 12v power. There are 1050Ti cards that do not have a auxiliary 12v PCIe power connector, and this means that, per spec, they draw less than 75W - but there are some that have a 6-pin PCIe connector. The 460W PSU in your Dell should have a lead which has two 6-pin PCIe connectors 'daisy-chained'; each of these is rated to provide an additional 75W to a GPU. I don't think I'd want to put two GPUs on that power supply, and I'm not a big fan of daisy chained power connectors at all, but you should be able to comfortably power one that has a single 6-pin PCIe power plug on it.
The 1060 warrants a little more scrutiny, because there are no models that strictly use the PCIe slot for power (in other words, all of them require more than 75W). Some have a single 6-pin PCIe power connector. I have a couple of the 3G EVGA SSC cards - factory overclocked models - and they require 8-pin PCIe connections (which are rated for 150W). Obviously, the overclocking adds a fair amount to the power drawn by GPUs.
If you're going the 1060 route, you might consider one of the cards that only uses a single 6-pin connector, as this will ensure the card doesn't require more than 150W (75 from the PCIe slot and 75 from the 6-pin connector) per the PCIe specs. These 6-pin cards seem to all come with single fan arrangements, which sort of stands to reason as the lesser power used will generate less heat. There are 6G cards that only require a single 6-pin connector. To be accurate, some have worried that single-fan designs don't cool well enough, but I'm not sure it's that much of a problem in a well-ventilated case (see below).
If you decide to go with a double-fan design, obvously it will cool better/run quieter, but consider it will also be bigger. If your chassis is tight, this might be a problem. Also, all the dual-fan cards I've seen have 8-pin connectors - and if I'm looking at it correctly, your PSU doesn't have one. The 8-pin cards will sometimes include an adapter for using two 6-pin connectors on a single 8-pin card. You'll want to make sure whatever card you get comes with one of these, or get one separately. TBH, as a trained electronics person, I'm not a huge fan of these adapters; more connectors also automatically means more resistance and more points of failure to me...but I'm super picky like that. Plus, in your scenario, the two 6-pin connectors are 'daisy-chained'; i.e., actually on the same lead from the PSU (if I've looked at it right) - this means that you potentially have twice the current moving through the same number of wires
...ordinarily, I'd try to avoid this, and only use one of these adapters if there are two separate 6-pin leads going back to the PSU. (They may well go the same place inside the PSU, but we're talking about the path to the load, not at the source).
Also, consider that any card with an 8-pin connector has it by necessity, because it draws more than the 150W a 6-pin connector and the PCIe slot can provide per spec. Now, the actual load on these cards varies and what I looked at wasn't very forthcoming about actual power used by the card itself, usually stating the size of the PSU overall in a system containing that card (which isn't too helpful, TBH). But, it remains that any 8-pin card, per PCIe spec can draw up to 225W at 12v
. Note they don't have to draw that much, but technically if they draw even a little over 150W, they have to go to an 8-pin connector per spec. If we allow 225W for a GPU, and 84W for the CPU, we're already over 300W. Considering overhead, power for other system components, and the aforementioned degradation-over-time, this scenario probably comes close to being too much for the 460W PSU you have.
Realistically, yours is pretty much a 'worst case' for adding an 8-pin GPU. That said, and in spite of all the horror stories I've described above, I'd honestly bet an 8-pin would run OK, but then you might have problems - yours is such an 'edge case', with all the factors considered. It could literally come down to how well your PSU has 'aged' and how clean and cool it's been kept since new. In any event, if you did have issues, you'd be stuck either returning the card or replacing your PSU, which you indicated wasn't part of the deal. (Incidentally, the PNY 1060 unit you cited in your post is an 8-pin model, FWIW.)
So, to summarize: 1050Ti is a good budget choice; 1060 3G and 6G are better still, but given the PSU constraint I'd stick with 6-pin models. In either case, because of the increased power and possibly minimal GPU cooling, I'd also consider very carefully how well your current chassis is ventilated. This is an area where OEM machines (Dell, HP, etc) sometimes suffer: They are great builds but aren't great for upgrades. You need to look carefully at whether you can add fans to your chassis.
Sorry this is so long, it's actually quite a lot to accurately cover.
Hope it helps, and I welcome any questions.