What sort of games are you looking to play on it? Do you want it to be future-compatible? 6600GTs are pretty mid-to-low range these days (I know, I have one). It's lower than the min requirement for Half-Life 2: Lost Coast and Quake 4 is very playable but it does struggle at times, something I didn't want to see
If you are looking to play some recent and unreleased games on it I'd start looking at a 6800Pro or maybe a low range 7 series card, that is if you can afford it. However if you are just looking at playing games like ET or not games that are fresh out of the dev shops then a 6600GT should be more than enough, I found ET and CS (and Source) to be more than playable on my old FX5200.