PC. I've built my own PC computer in the past (twice) for about 1/2 of what I would have bought a new computer for and gotten more power because of it.
This.
I have nothing against Macs, apart from their price. They're good, solid machines, and better for quite a few applications. For some people, they're a very good choice. I'm not one of those people, though.
I
like building my PCs, choosing each and every component, and having near-total control over everything that goes into it. $800 + a single evening of enjoyable labor gave me the same quality hardware I'd need to pay
well over $2,000 for, if I'd bought a Mac. I also have more upgrade paths available, have a wider variety of software at my fingertips, and can get my hands dirty in the OS if I really feel like it.
Also, while Vista was a train-wreck when it was first released due to some poor decisions and horrendous driver issues, right now it's a rock-solid, high-performance OS with immensely better security than XP. You just need a modern system for it. Windows 7 promises to be even better, right out the gate.
Anyway, building a Hackintosh also sounds like a lot of fun to me. I might do that at some point in the future, too.
-O