I realize I'm late on this topic...
with audio recording/processing, ram is not as big a factor with regards to system performance(of course, this can also be app dependant...not the case with Digital Performer).
of course, I would still have at least 768(256 on board + 512 dimm), but that is for general OS performance.
we use a 867MHz G4 Powerbook and routinely run 20 channels via firewire 400 daisychaining a MOTU828(8 analoug), MOTU828mkii(10 analoug + 2 spdif) and a Glyph 7200rpm hard drive with no jitter/artifacts.
when we need to sync slocks we can use either the 828mkii as the master, or slave that to a external...