Parallel's runs Windows in a virtual environment, as a process in OS-X. The nice thing is you can switch between Windows and OS-X the same way you switch between any apps [in OS-X]. The downside, is it currently doesn't support h/w accelerated graphics (3D).
BootCamp allows a dual-boot solution. You select which system you want to boot into. The drivers support most of the hardware, including the GPU, so games run full speed. BootCamp is in beta, however, and there is no Apple support for it. From what I see, it mostly works fine.
I haven't tried either, as I'm waiting on my 17" C2D.