FWIW, there's an easy way to solve the Mac vs. PC vs. Linux debate: you have software for which platform? If you don't have any, THEN you have a question. The breakdown is, essentially:
* If you can live with non-industry standard apps that work almost as good as the industry standards and are almost compatible with the industry standards, you have virtually no budget AND you're either an expert or know an expert, Linux is for you. (No whining about WINE: it's difficult to set up.)
Linux is an awesome setup, particularly Ubuntu, but it is difficult to set up. After it's set up properly, it'll work perfectly forever. (Also, if you want to do Linux and you don't have a clue about it, have your expert build a machine or buy a Dell.)
* If you are more interested in taking the long view on total-cost-of-ownership, you're not a gamer and you've got more money up-front, a Mac is for you. The TCO comes from the fact that it requires fewer support calls, is easier to learn and the hardware tends to break down less than on PCs. (I've used Macs, professionally, since 1985 and I've worked FOR a fruity company and several Mac-only shops, so I have an informed opinion.)
* If you want to be compatible with 99% of the world, out of the box, you don't have tons of cash and you're either a techie guy OR have a support staff -- or are a serious gamer -- Windows is right for you. I recommend Vista to people that don't have much (or any) computer experience or are serious gamers that need DirectX 10. Otherwise get XP. It's faster.