From what I can get out of Digidesgn, there is no immediate plans to release a version of Pro Tools that works with OSX. So it seems there is no rush to upgrade your CPU. I am running ProTools Mix+ on a G4 466, and will be doing the upgrade to HD soon. There doens't seem to be any compelling reason for me to get a faster G4 at this time. I'd like to get another year's use out of it - who knows, by then we may be talking about G5's.

My philosophy on computer upgrades is don't do it until your software demands it, or your work rate and efficiency is noticeably suffering from lack of CPU speed. Unless money is no object - then, by all means, upgrade every six months or so! :p

Digi's pretty clear about their intention to support OSX (and WinXP) in future PT releases. However, I've seen nothing specific about dual-processor support (unlike some of the other MIDI/audio programs out there) - so that particular system factor may not be your main priority (if you buy a new CPU between now and then). Disk performance is one of the main things you want to keep an eye on - your current setup might get clobbered when you start to wrestle many channels of 96K/24bit audio - check out Digi's compatibility section to see if your current discs even make the grade. (Note that they now support certain FireWire drives -including their own!- for audio record, even at hi-res on HD systems.