If you’re a big gamer, you’re probably going to be most happy with a Windows machine that’s properly spec’d out for gaming (good video card, fast hard drive, and a good CPU). While you can certainly run Windows on the new Intel-based Macs, you may find less upgrade paths for the Windows side of the equation that will still work without consequence on the Mac side. (ie you can’t just upgrade the graphics card and expect that it’s going to continue to work when you boot into Mac.) On top of that, to get the proper gaming experience on the Mac, you’re probably going to want a Mac Pro instead of the iMac, and that can get pretty pricy.

If you get a new Windows machine with Vista, you’re certainly going to have much better protection from viruses and spyware than you had on XP…and if you follow the basic common sense rules of online computing (don’t click on links or attachments in email, avoid warez sites, etc) and protect your computer with anti-virus and anti-spyware software, you should be able to stay reasonably safe.