I think it's pretty hard to go wrong now days. I personally use Nvidia and have been for a little while, but as I said you can't really go wrong with either AMD/ATI or Nvidia cards. Nvidia cards should work fine in any AMD system, it's just an AMD system will mostly these days only accept crossfire I believe, so if you want multiple cards one day chances are you'd best look into ATI cards, although I'm not certain SLI and AMD don't go together anymore.
8800GTS 640MB would be a decent buy although I think the real winne right now for value is the 9600GT. I'm not sure on the prices exactly but I think they're in the same sort of range. Not too sure about what ATI/AMD offer in the same range.