The PCI-e bus isn't even being utilised properly. I have a 7800GS on an antiquated AGP slot that will compete with, and propably beat most stock PCI-e 7800GTs out there. The AGP slot has a fair amount of life still left in it, the move to PCI-e happened WAY too soon!
Its the same as when people go on about dual x16 PCI-e slots in SLI mobos, and how you'll only get half the performance increase in SLI if you have dual x8 PCI-e slots. That is plain bull****, since most cards, including the 7900GTX, don't even use all of the bandwith PCI-e has to offer. I'm sure that the 7900GTX in AGP format would be very competitive compared to its PCI-e counterpart!