>Right, but while in theory there shouldn't be compatibility problems, some PCIe v3 cards have been reported to not work in PCIe v1 slots.

Its more complicated than that.
I haven't finished discussing this with AMD, and using later cards in a v1 slot will probably need some experimentation.
As I've said before, the Radeon HD 6570 will work in a G5.

BUT, there actually may be problems using v3 cards in v2 slots as well.

This is a partially quote from a message I received from AMD tech support today, "I believe that the HD7000 series GCN1 GPUs should work with the older PCI-E 2.0 revision Motherboards, however you may encounter issues if you attempt to use anything newer as we have had reports from customers that had compatibility issues when using the new RX400 and 500 series with PCI-E 2.0 motherboards."

SO, NXP lists the PCIe controllers in the P50xx as PCIe 2.0/3.0, but what has Aeon/Varisys implented?
If those slots ARE only v2, they may not work with future AMD based video cards.

AND, does AMD's statement about v2 apply to v1?
If so, the G5 could potentially support the 7000 series cards, if not the 6000 is pretty good (and probably the best we are likely to get support for in the next driver upgrade anyway).

Its a story that is continuing to evolve, but I personally am moving forward on putting together an X5000 (although I do have a Quad 2.5 GHz G5 sitting in the wings).