OK Peter,
Sorry for mistaking your situation. My point remains, though, that buying a PCI-e Mac is not a great call right now. There are very limited options for soundcards and, obviously, no compatible UAD-1s. It's cutting edge stuff and those who take the plunge have to be prepared for some hardship until the rest of the world catches up... and it's gonna take some time for all the crap to shake out of the new drivers and hardware.
For you, if you want to upgrade from your G4, why wait for the Intel Macs? Do you really want a 1st generation Mactel? Why don't you seek out one of the discontinued dual 2.0 GHz G5s with PCI slots (not PCI-X or PCI-e)? Yes, they're discontinued, but I think you can still find them new-in-box. I got mine recently at at one of those giant electronics stores. And there are PCI slot G5s on Ebay right now... don't know if they're new-in-box, but they might be. Check it out.
Also, you might want to keep an eye on the refurbs at the online store at Apple's site. There are no PCI G5s now, but they might show up there. I've seen 'em before. Maybe they'll show up again.
I just think that if you get a PCI-e Mac now or a 1st gen Mactel when they're available, you're just trading one set of problems for another and it's gonna be a while before those machines start to really pay off. The PCI G5s work well, they're faster than your current machine, and they're totally compatible with UAD-1 cards.
Good luck.