I just had a look at the 7950 and apparently quite a few people have had issues with the drivers from the card? And why go for a different PSU?
Yeah the drivers always been an issue with Amd cards but apparently, since the new Catalyst version (12.1 beta i guess) there's no more problem and everything works great. You can take a GTX 660Ti if you want but if i had to rebuild my pc, i would try a Radeon card.
The 650W power supply will do the job but prefer getting an extra 100W for headroom and to be sure your Psu don't work too hard because it will die faster.