Just get a 660 gtx, the xfx 450w will do for a 660 gtx, but not with two 550ti or 560 ti + 550ti for physX. http://www.newegg.com/Product/Product.aspx?Item=N82E16814130825
A single 660 gtx is better than two 550ti or a 560 ti+ 550ti physX.
Ah that is a much better card, I will be going for that instead of 550/560 ti but then I Found this: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-4.html
, it is a Radeon 7850 2gb which cost less than the gtx 660 but im not sure which one is better.