This is really a Ford vs Chevy discussion. Some folks have had no end of issues and quirks with ATI and some have had the same with NVidia. They constantly leapfrog each other in performance, so it's really down to price, heat and power use.

I would be leery of running any GTX460 or higher NVidia on a 650W PSU, IMO. I've run a 5850 on a 600W PSU without any PSU-related issues, though I'd probably go 750w+ just to allow for extra HDDs or other additions.