Don't buy anything until after Tuesday. That is when Apple is supposed to announce a new laptop geared for education and college students. Check their website Tuesday afternoon. Their latest computers have been very competitive price-wise, and most music students use macs. (It's possible the announcement might get delayed a few days more):
http://www.thinksecret.com/news/050609macbook.html
Get as large a hd and as much ram as you can afford. Then get a firewire audio/midi interface. Check out MOTU and M-Audio products, and possibly Edirol.