Mmmmmm.....G5
I ended up getting a 2nd 80Gb internal HD for my audio files, and am using the other HD for OS and all appz. You might want to try something like that if you can afford it.
But yeah, generally I think OS and appz on one partition, and audio files and projects on the other partition. I did come across a tutorial for optimizing OSX for audio, I will try and find it for ya.
You will love the G5, they are solid machines!
I havent upgraded to Tiger yet though, dont think I will cos it will mean having to buy new software/upgrades, and I just dont trust Tiger atm
Good luck!