Having all three (but having studiously avoided Vista…), I’d say that it really does not matter much. What you’ll need is a very good graphics card and a very powerful CPU.
Now… I personally find that Apple builds that sort of top-end hardware into their stuff, and they provide a good bumper-to-bumper warranty. Since they do serve the graphics community, things like gamma-correction are paid attention to. So, as it happens, I do all of my design work on the OS/X machine, which also happens to be the most powerful machine in my little “shop.” (My wife and I are busying away on the XP box and the Linux box, respectively, while the Mac does some early-morning compositing. From time to time, all of them serve as part of an overnight render-farm, and you just can’t tell where any of the frames came from.)
I happen to recommend and to prefer the Mac. But in my profession I’m bouncing back-and-forth between everything.
Wanna use Windows? Nothing at all wrong with that much-lampooned system… Just two caveats: - Buy a “real” edition, not the “home” one, and re-install it on your box. - Learn about user-accounts and “limited users,” and also Microsoft Backup … which you probably have but no one talks about.
And for any box: - USB and/or FireWire external disk drives … huge ones … can be had at a department store for sometimes less than $100. Buy one. Use it.