The 7970 would be the best choice instead of the 7950... BUT because its on sale for that insanely cheap price, buy the 7950!!!! as theres little to NO difference between the 2, or at least no difference that i would consider..
I'd recommend you to get a 650w+ just to be sure, and you'll have more headroom for overclocking. And the 7970 is better of course, but you'll be able to run ultra with no problems on the 7950, and you can overclock your GPU, that's a positive thing too.
Note that it does not overclock as well as either of the AMD cards, but it's overclocked performance will still be as good as that of the HD 7950.
It's also a bit more power efficient.
That sounds awful because you're going to want the OC options, it would only be logical to go this way if he wanted Physx or whatever.. and even thats not worth it.. the 7950 in this guys case is a good deal, theres no reason he should go NVIDIA... BUT the current card he has does make me question his CPU worth for those GPUs... an PSU capacity