On my MBP, I've got a BootCamp partition (with WinXP), so I can boot into a thoroughly unsimulated Windows environment, and I also have VMWare, and when initially setting up VMWare I chose some option that made it construct a special VMWare "image" that points at my BootCamp partition (sorry, don't have the right terms at hand). I've also got a "normal" VMWare image with WinXP as well.
Anyway, this makes it so that I can: 1) run Windows in an entirely standard VMWare environment (with that cloned image that's sitting on my Mac partition as a 10GB-ish folder) and get all the standard VMWare goodies, like snapshots and suspending the VM without having to shut it down, or, 2) I can run the "hybrid" BootCamp-under-VMWare image in VMWare, under Mac OS X, and have access to my whole Windows partition (though you have to shut down the virtual machine before quitting VMWare), or 3) I can reboot into Windows and have a no-gotcha no-compromise WinXP environment, if some game demands more performance than VMWare can muster.
I really like the versatility this setup affords me. I believe you can do similar with Parallels, but I only have experience with VMWare.