I just had a **** of a time trying to decide whether to get an AGP card since my board right now has no PCI express slot. I decided to order a new cpu&mobo since fellow TF members have mentioned more and more companies are phasing out AGP so to make it easier on myself I got a mobo with a PCIe slot. If you ever decide to do what Ive done go
and you can get a pretty good deal.
However, if you don't feel like upgrading your motherboard to PCIe (which will force you to get a PCIe card, obviously) there are several deals on newegg aswell for the 7600 gs. And yes I think upgrading from your 6600gt would be..it is no longer top of the line and upgrading would help you alot.
btw, what games do you play?