I will be running games ranging from Starcraft 2 to games like battlefield 3, I want to be able to play at high graphics on a 1920x1080 resolution, what do you think would be better running with an i5 2500k and this motherboard: https://www.aria.co.uk/Products/Components/Motherboards... 1155 Z68 (B3)/Gigabyte GA-Z68X-UD3P Intel Z68 (REV B3) Socket 1155 DDR3 PCI-Express Motherboard ?productId=44487
Tough choice, both have upsides... I'd reccomend the 560ti though, so you have the raw power now and possibility to upgrade greatly later if you want. The 550's would probably give you an edge on performance now, but with SLi you run into a lot of other issues and bugs with certain games that you would avoid with the 560ti... plus the 560ti should max everything, or pretty close, that you would want to play now.
As for the power supply, I've recently learned how incredibly sufficient 750w is! I'm running a 6-core i7 overclocked, two 6870's overclocked, three drives, and now liquid cooling, with a bit to spare... So I wouldn't worry about the psu too much until you try to sli 560's!
I have to agree with all of the other comments. I strongly agree about the 550's in SLI. I would recommend not to do it even in SLI they arn't that strong at least not for the games like crisis, BF3, Skyrim, etc. You won't be able to play on high at all only med. I know this because I have two of them in SLI and they are both OC'd.
GTX 550Ti SLI have more pronounced micro-stuttering. And going for this route you kills the SLI future upgrade path.
Going for GTX 560Ti-448 would make harder for SLI in future since it's a LIMITED EDITION card.
Go for the GTX 560Ti
The micro stuttering gets progressively better and better up the card level. I find that the gtx 570 and 580 are when the stuttering gets reasonable, but by then you can get away with just the one card.
You might be right about the gtx 560ti-448 being harder to get later, but I think you'll find that they aren't that rare now. People will be wanting to replace them when we see what comes out next for the GTX card series, so used card will still be obtainable, probably for cheap.
I would hate to buy anything electronic used. You don't know how many hours has been put on the card. You could buy the 560 ti 448 core used, spend a lot of money for it to be at it's end, and only get to use it for a week of two if that.