I was looking at 512mb card on ebay the other day but after reading a review it was kinda obvious that the 512mb is just marketing. They recommended a 256mb card with a better gpu.

Also I upgraded from 512mb to 1024mb a couple of weeks ago and saw little improvement. But maybe my old graphics was working at full pelt with just 512mb ram and wont scale further even when I overclock the fsb.