AGP is just a subset of PCI, not including the bus-arbitration (turning it into a point-to-point protocol rather than a packet-switched protocol) and some new addressing modes which have more to do with driver support than with the bus interface itself. (the cards don't touch that end, so weither it's there or not is irrelevent to the cards for the most part) AGP 2x is pretty much the 64-bit version of PCI on a 66Mhz bus w/o the junk that's not needed. Not hard to use firmware and a little bit of glue logic to turn any PCI bus into an AGP bus. PCI-X offers even more opportunities, as the availible bandwidth is much higher as well as the not-needed elements from PCI have been removed for the most part.
