I would be willing to try putting the one 2.7 CPU in this 2.0 since there is only one of them and it may run quieter and I get the faster bus as a bonus? Do you have to stay in the same family (PCIX and PCIe) of systems? Does the CPU dictate the clock or the bios?
Interestingly the PowerBook 1.67 G4 seems to play back media better than the G5 2.0.
