Please see my comments to your concerns below, hightlighted in Orange:
Brand: I'm thinking about either a sony vaio or powerbook
Keep in mind that a Sony Vaio means taht your are getting a PC, and that means you can only run software made for a PC. If you get a Powerbook, that means you are getting a Macintosh, and that measn you will only use software made for a Macintosh. They aren't interchangeable. If you are going to college, find out in advance what they prefer you to have.
Processor Type/Speed: I don't know if it should be a pentium or what
Faster is always better. My suggestion would be a Centrino processor PC laptop with a 1.7GHz or better CPU.
Operating System: My guess is I should go with XP
Yep, that's what I'd do you don'r NEEEED anything more that XP Home, but XP Professional could be handy if you plan on taking any computer networking courses.
Memory (RAM): at least 512 is my guess[/QUOTE]
If you want to do music, I consider 1GB to be the minimum. If you can't get the 'rents to spring for that much, make sure you can upgrade later.