If you plan to do any sort of work that requires anything more than modest demands of your graphics subsystem, use BootCamp. Parallels cannot take advantage of hardware acceleration, whereas BootCamp can.
If you're going the BootCamp route, I would hold off until Leopard. For now, BootCamp is technically beta which means Apple has zero liability/support/responsibility for it.
When Leopard comes out, they may very well disable the beta versions floating around out there and tell you to wipe the XP partition and start over from scratch - with a newly purchased 10.5 retail box.
Pubb