I just wanted to pop in a quick update on this, mostly for Plowman's sake (a fellow original G5 dual 1.8 owner).
A couple of days ago I decided to do another clean install, but this time to a new drive. The idea this time was to make a totally dedicated audio boot, not an account, but a boot. I did a lot of searching around for 10.48 reports and found a *lot* of people having problems. So, I partitioned my drive into two equal partitions - one for audio only and one for graphics, web design, and general crap - and installed a new system to the audio partition. I downloaded and ran *only* the 10.4.7 combo updater, then did all the updates. I re-installed only my essential audio apps. And yes, I am back to the snappy system that prompted the original "like a new machine" comment. Victory!
Unfortunately, without going through installing all those extra apps one at a time, I will probably never know what exactly brought the system down. But I will note that the one thing that made me absolutely certain that something was *very* wrong is that when playing back audio files from the Finder, using the Preview column, i would get a delay of around 700 ms before playback would start. This was also the case for locating to different points in the file (still in Finder) and for iTunes as well. So, Plowman, if your system is doing this then you probably have the same issue, conflict, whatever, that I had. I wish I could tell you what particular app brought the system down, but I can tell you this it appears *not* to have been an audio app.
cheers,
J.