pci-express is better, main feature it is a faster connection it is 16x where agp is like 4 or 8. I have recently seen a lot of ppl having trouble with the 6600 gt for some reason, i do not have it myself. i would say if you want to get a card that expensive you might as well just get a 6800 or even 7800 if you really have the cash. now the 7800 only comes with pci-express or at least the BFG version so if you dont have those slots you need a new mobo i have found a combo pack of a evga 7800 GTx Oc that includes and nvidia evga motherboard for about $550 or so. The only problem is it is amd not intel and you have an intel processor. So in that case i would just get the 6800 it is better than the 6600 and seems more reliable, and to use it you wouldnt have to upgrade your system unless you want to go pci-express to keep up with the times.