Well i personally would go with the 7950.To be fair though both cards perform almost the same unless the game is specifically tweak to favor one of the architecture.If you play at 1080p both will give you good result in most games.The GeForce GTX 660 Ti has core speeds of 915 MHz on the GPU, and 1500 MHz on the 2048 MB of GDDR5 memory. It features 1344 SPUs as well as 112 TAUs and 24 ROPs.
Compare that to the Radeon HD 7950 3GB, which comes with a clock speed of 800 MHz and a GDDR5 memory speed of 1250 MHz. It also features a 384-bit bus, and uses a 28 nm design. It is made up of 1792 SPUs, 112 Texture Address Units, and 32 ROPs.The 660 Ti and 7950 perform basically the same at stock up to 1080p resolution, trading blows depending on the game. The 7950 most likely will OC better and it will murder the 660 ti at high resolution (and has way more compute power), the 660 Ti gives you CUDA , PhysX, TXAA, FXAA, ADap. Vsync etc and lower power draw. Both are solid buys.If you still can't decide which card to take try looking at the games that you play or specific features that you want.If cost really matter to you then you might want to go with whatever is cheaper to you.As far as yourset up being compatible i couldn't say without knowing your exact power supply.