An audio computer will need a dedicated audio hard drive (7200rpm IDE), lots of memory (512MB or more), a good quality soundcard (Audiophile, MIDIMan Delta, etc), and a fast processor (many say Intel are best, I've always had AMD)
I have a 1.47GHz Athlon and have no problems at all running loads of plug-ins, although I dont use my PC for recording audio so cant comment there. Since I upgraded my processor and my memory to 640MB, I really have not had any problems on my PC (Windows XP)