MS PC or Mac! I have PCs, stacks of them, I stuck with XP when I bought my video/photo system, because it runs the same software. Compatibility. You can still get XP until June 2008, don't get Vista... Yet! Give them another year in Beta to get the drivers supported and get the bugs out.
I have Canon lenses, I stuck with Canon. Nikon makes great cameras too. If you had all Canon equipment, would you switch to Nikon, because they made a better TTL flash or Macro lens? Then why switch computer systems? If you are a Nikon shooter, would you switch to Canon because the new 1D Mark III has a GPS module?
Macs are great, fast, and every Mac user will tell you how they are the best in the world. (all 5% or the PC users in the world, who can't stop telling people how wonderful their Macs are for some reason.) Every wonder why there are only 5% of the PC users in the world running Macs, but every time you write a message, you'll get a flood of Mac defenders, writing that you want a Mac?
Go price Macs and I think you'll have your answer. If you can afford one, and don't mind buying all new software for everything you do, you'll be happy. Macs do run a version of Windows now, which is amusing, because if Macs did it all, why would anyone want to run Windows on a Mac?
True, Macs are better, faster and designed for graphics. If that's all you are going to do on your computer, and you have money to spare, then buy one. If you want to run things that the other 95% of the world has available, you might want to stick with a Windows PC.
Is security a big issue? You don't run behind a firewall that's built into most routers? Buy a Mac!
Then I can throw in this wrench. Ubuntu or Linux are pretty good, offer all kinds of free software, and GIMP photo editing software, FREE! Want to learn a new OS?