It all depends on what you're going to be doing. 2 programs that are very popular right now are Propellerhead's "Reason" and Cool edit Pro 2.0. Both are around $300. 128 tracks on cool edit and Direct X friendly. Not sure on Reason. If you don't need this many tracks, Cakewalk Guitar tracks is a nice 8 track program around $50.00 last time I looked. Very stable.

Good luck.