Get a solid desktop. (we like the Dell Vostro's running 2 gigs and Win XP)
Get either a good sound card (Lynx, Delta-44 or CarDeLuxe etc) or a solid USB or firewire interface for audio
We love our wireless trackballs for editing.
BUT invest in a standalone external hard drive to hold all of your audio. The operating stuff including the temp directory for editing etc for Adobe Audition 3 can stay on the "C" drive, but save all audio, mutitrack settings etc to an external drive. Keeps the operating speed snappy, and if the tower's HD crashes you can still keep your audio.
These have gotten so cheap that we now replace these external drives every year and shelf the hard drives by year (2006, 2007 etc...) and if we need to go back, we just pull it down and fire it up.
I needed a track I used on a 2005 car spot this past week, and found it in about 2 min.