If you have Leopard then all you need is a Windows XP SP2 or Windows Vista install disc and Bootcamp (included with Leopard) will guide you through the rest, basically what bootcamps does is install Windows XP/Vista on your computer and the appropiate bootloader to let it boot, Windows XP through Bootcamp is the same thing as Windows XP on any other PC.
Parallels Desktop is a virtualization app (not emulation) that lets you run Windows XP within the OSX enviroment, performance is generally very good but it can be quite the memory hog, 3D performance has not been nailed down right properly so Parallels is good for running 2D games and non-demanding applications like Microsoft Office.
Bootcamp is free minus the cost of the WinXP copy and Parallels has a cost (can't remember now) and a free trial.