We just did a Upgrade from one of our systems to a MacPro Dual 2.66 XEON.

All work out OK, no issues, the Catalyst 4 is a Universal Binary software, so works without any problems...

the only thing, is the new mac is a PCI-e slot cards, so any boards you may have on your G5 will not fit on the new computer, you need to buy new ones, I do not know which components you hav, but like in my case I needed to get a new Blackmagic card PCI-e.

Works OK... the best thing on the MacPros is the Inside DRIVE Bays, we did a RAID config SATA2 with 3 Drives, we are reaching real 180 Mb/s

Gian