If you don't care about gaming or anything super intensive, use Parallels Desktop so that you can run Windows concurrently (and much faster than VirtualPC) on Mac OS X. If you don't mind dual-booting and want as much speed as possible to play the WIndows-only games out there, use Boot Camp to create the drivers disc and the partition and install Windows. Just remember that Boot Camp is still in beta, so make sure you back up your files.