what do you plan on doing with it?
for music production, i'd go with a desktop because they're cheaper and more expandable — i just got 2.33 ghz PC with 2 gigs of RAM, an M-audio soundcard and a 20" flatscreen monitor for under $1000 u.s. —- but if you want something for playing out, i'd just go with anything that has at least a gig or 2 of RAM and 1.8 ghz processor speed. you should be able to get a fairly inexpensive used one with those specs.
otherwise, i guess you could be one of "those" people and buy an apple laptop, which are supposedly the greatest thing ever in the history of the world (assuming you are made out of money).
i'm no help whatsoever.