ATI have come out with a convertor now that will also let there higher end PCI-E cards play on AGP computers without any problems, since at the moment, PCI-E is not any better than AGP.
Though I'm sure they'll show there generation gap sometime. This will prove to be a good step for ATI.
As for the Geforce 6600GT, this is a great card for todays games and Money_man is right in saying so
If you can afford along the lines of a 6800GT or Ultra, you'll be playing games for a very long while longer, even if new games come out. You'll still expect to play on high res's, high settings.
Just check around this forum for great news on the 6600 chip
I would advise though to get a mid-range card (6600GT is mid-range) and then you can simply update if your computer becomes too slow again, as its a cheaper option.
Good luck with choosing