Dont worry about having a 256mb video card, most of those cards dont use the memory properly and having it is paying more for nothing. The 6600gt 128mb outperforms alot of 256mb video cards. Trust us, you want that for a budget card or a 6800gt for a kickass card. The 6600gt will play HL2, BF2 fine at most settings high.
Another option is a plain 6800 128mb which is in between the above 2.