Quote from Hercanic
Not sure if you're an Nvidia fan, though, as a number of AAA games are designed with GeForce in mind. When I was trying to decide, the benchmarks were close enough, so I went by physical difference. The GeForces are larger and require more power, while Radeons run hotter while idle but not as hot as the GeForce at full load. I figured physical size couldn't be changed, so Radeon won there. More power cannot be changed either, meaning a higher electric bill, so Radeon wins again. Finally, hotter on idle seemed easily remedied with good cooling, while hotter on load would output more overall heat and be harder to manage, stressing components more, so once again Radeon wins.
Also, between 512MB and 1GB, according to various tech reviews and forums, the performance difference will only be noticable at higher resolutions. 512MB is fine up to two thousand something by a thousand something. Beyond that is where the 1GB will help. So if your monitor is less than 24" and you have no plans on upgrading, you'll be fine at 512MB.