Do you have the software already?
A Mac is the one to go for, although remember that if you get an Intel Mac, older Mac applications which have not been updated to run on the Intel machines will run slower than those which HAVE been updated.
I'd get a MacBook. You get GarageBand with all new Macs, which is basically stripped down Logic in a fancy suit and loads of built in instruments and effects. It's actually much MUCH more powerful than it first seems. When you first use it, you'll think it's more of a toy than anything, but get used to it, and you'l discover things you can do with it which actually make it very VERY powerful.
If you can't afford a MacBook, get a second hand iBook G4 or PowerBook G4. They're very cheap at the moment considering their performance and usability.
I'd still try and get a MacBook though. If the cost of one makes you cringe, then remember that it will stand the test of time better than a PC as long as it's looked after. My G3 iMac runs Tiger with absolutely no issues at all, and Reason (a synthesizer studio package) actually runs relatively acceptably! It begins to run out of processing power once it gets to 3 or 4 synths, but that's surprisingly good considering it's 10 years old!