PCs suck for audio. Every Studio in the world is Mac based.
not really. Pretty much every studio in my hometown of Murfreesboro, TN (Just outside Nashville) are PC based studios. Of course, most of the guys who own these studios are in their mid to late 20's and have grown up being comfortable with PCs...and besides, any studio that is Mac based is probably a host based system and performance of the computer itself doesn't even matter.
I personally run an XP Pro machine using an AMD 2700+. I built it myself from scratch specifically for doing audio (oh yeh, and gaming - seperate hardware profile for that). I have had ZERO problems with any of the audio software on my system for one specific reason: I built the machine to record and mix audio; there is no crap on my system that could potentially cause problems. Macs come with all this iMovie, iPhoto, iThis, iThat, iThink iMgonnascream. Why can't I get a cheaper Mac that doesn't come with all this extra software I'm never gonna use? Or better yet, let me build my own friggin Mac!
Of course, when it comes to laptops, you can't really build your own so you're options go down pretty quick. My advice: buy the fastest laptop you can afford (keeping in mind that more gHz doesn't necessarily mean faster performance), format the hell out of that hard drive and only load the software you'll use and disable any hardware you don't need (modems, bluetooth, wireless cards, etc.). the Pentium M processor (not to be confused with the Pentium 4 M) would be your best choice if you can run off AC power. If you're forced to run off battery, try to get a laptop that has a regular desktop chip in it, as your performance won't take such a huge hit when not on AC.