It really depends on what happens in the processor arena. If the high end of the CPU market quickly transitions over to dual core processors that are only available in packages that typically require you to team up those processors with PCIe motherboards (LGA775, s939 and eventually M2), then you will likely see the market for high end AGP cards evaporate. The reason for this is that in a year's time it will become increasingly hard to find s478 and s754 processors that won't bottleneck the new high end cards (R520 and G70/80). Sure, there will probably be a few oddball boards that use the new sockets and chipsets with AGP, but those will probably fade away over time as PCIe becomes more entrenched. Eventually the card makers will start cutting back on AGP production, which will then cause the AGP card prices to jump and make them even less attractive compared to an upgrade to a PCIe mobo.
I'm actually willing to bet that AGP will disappear before PCI does, simply because Dell is still to this day cranking out the cheap Celeron boxes with no video upgrade path beyond a PCI add in card. Even after the full conversion of the gamer market over to PCIe cards in a few years, you'll probably still have some stripped down chipset for the budget PC market with onboard graphics, a silkscreen of where the x16 slot would go if your computer didn't suck and a single lonely old school PCI slot below the x1 slots that will keep PCI graphics alive for a few more years.
The really big thing that was holding back adoptioon of PCIe at first was the suckitude of the Prescott P4s, which was your only game in town processor-wise when PCIe was introduced last year. Now that the nForce 4 mobos are going mainstream and the s939-only Athlon X2s are coming, you soon are geting to the point where it would be just silly to consider spending $500 on the successors to the 6800s and X850s without doing a PCIe system upgrade in the process.
