I'd say nvidia, but I'm not sure about the cards you listed, do you have any links
I know the 6600gt is a great "bang for the buck" card but I have no experience with ati and I am not sure what good that card is...
and If you decide to get a 6600gt, just get the 128mb version, save some money, that gpu doesn't need 256mb to run well, (personal experience)
and when It comes to ATI vs. Nvidia
if you were to compair them based on drivers, Nvidia would win
and if you were compairing them based on price/performance ratio, Nividia would win...
so.. my pick is Nvidia (and a good manufacturer is eVGA)
what kind of price range are you talking about?