I've been lucky (But then again, I'm not in Mac land so my card drivers exist) that my SCSI rigs have moved without so much a problem, some older drives (Specially, as much as it pains me, some of my older sample libraries) refuse to play with SCSI-160 and upwards... fortunately there's ways around that.
As for Firewire, at the moment I'm using FW drives for backup, I wouldn't dream on recording to them (Then again, I'm on Windows, and Windows and firewire have a weird love/hate relationship).
That's one thing that's bugging me of late, seeing that Digi is all too eager to get the Windows DAW market away from Steinberg and Magix, why not a Winblows PC for your HD rig? So far the reports I've had of PT on windows is that it's as unstable as PT on the Mac (Whatever that meant, not sure if it was an insult or a compliment).
What I do believe, is that if you go for a dual G5, make sure everything else you need is covered, three meagre PCI-X slots are not nearly enough, and expansion chassis are expensive and unreliable.