What kind of games did you want to run? Be careful since neither Parallels Desktop nor VMWare Fusion support DirectX 9 or 10. They are both stuck supporting DirectX 8.1 with Parallels working slightly better than VMWare with game compatibility. If the games you want to run rely on 9 or 10, you may have to resort to Boot Camp to play them as Boot Camp is a fully operational Windows computer.
Parallels has stated they are working on DirectX 9 compatibility that will be released as a point release for version 3, so if you buy 3.0, you'll get it for free when it's ready. VMWare is very tight-lipped and has not stated what they're working on.
As another warning, Microsoft's EULA (End-User License Agreement; yeah that thing nobody reads and clicks past immediately) permits only Vista Business Edition or Ultimate Edition to be run in virtualization as Parallels and VMWare work, so Vista Basic, Home, and Home Premium are out. Windows XP has no such limitation. Boot Camp is not a virtual machine, so the Microsoft license restriction doesn't apply.
