> What do you get?

PCIe v2 (vs. PCIe v1).

> it might have faster memory (haven't looked at the exact numbers)

See comment #666.

> If there is no way to shut off, or put to sleep, the unused 3 extra G5 cores,
> running MorphOS3.10 on the quad G5 will be very energy inefficient.

Yes, that's why the lower clocked dual-core models of the PCIe-based PowerMac G5 would surely be worth considering.