OP. Make sure that at what ever cost, when building your PC, DO NOT BUY VISTA! Get Windows 7 or Windows XP. I still use XP SP2/3 (not quite sure which one I'm on) today and I've only ever had 1 problem. XP was the last great OS from Microsoft, and 7 is the latest greatest OS.
Vista was basically a for sale beta version of Windows 7 and as you know it was riddled with problems. Also, Adobe CS5 is made to run on 7 if you plan on upgrading, CS4 wont run on 7 with out a hassle. Due to the fact that CS4 was built to run on XP/Vista (it runs way better on XP though).
As for the Mac vs PC debate.
Mac's are a waste of money IMO. They give you little advantage out the box over a PC for a good $600 dollars more. Also given the fact that if you really wanted to, you could duel boot OS X on a PC and run the few programs that work better on a mac than a PC with out spending the 1300 bucks for a mac book..... BUT truthfully, the only reason I'd ever buy, or recommend a Mac to someone, is for audio recording or production... Pro Tools is GOD on a mac. All of my DJ friends, (DJ Dust, DJ Ethical, DJ Alphatrion, DJ Klever) all Atlanta Local's but are well know around the world, use macs, and they do some damn good mixing. So I leave my views of mac at home when I'm around them.
I work in a local music venue as well, and the quality of sound we put out due to the mac hooked into the sound board astounds me.
So the only thing a mac has over a PC is Music compression and recording software and the like.
But thats just how I see it.