What I have done, which works well for me, is this.
I first made a Boot Camp installation of WinXP, I gave it 20GB of my MBP's 160GB, and installed it there. Allowing full native use of WinXP when required. I then purchased Parallels, and installed that. Parallels is able to use the BootCamp installation of WinXP meaning I have one copy of WinXP that is usable natively, or virtualised within Mac OSX.
As well as this, I use CrossoverMac to run Counterstrike Source, and other Windows apps within OSX.
Hope this helps,
Charlie - f