Tough to really compare those two cards, as the 9800 is a second generation card and the 6600GT is a first generation. The 6600Gt is the obvious winner and will perform better in most everything, but the 9800 is still a really good card and isn't very far behind.
Go with the 6600 GT, as he says it will outperform the 9800. However, if you have a bit of extra cash go with a 6800 GT, the small price is more than worth the incredible performance boost.
