If you're particularly indecisive about your regular operating system, or just want access to all three for testing purposes, Wired's How-To Wiki has the details on getting a triple-boot system (or beyond) working with free software tools.
Photo by Foskarulla.
It's not for those who like to avoid terminal commands, and it's definitely not meant to be crammed into a half-hour lunch break. But Wired's guide is fairly thorough, assuming that burning an image to CD is base-level knowledge. You'll use the rEFIt tool to set up a multi-system boot screen, the Lifehacker favourite GParted Live CD to smooth things over once your hard drive is carved up into partitions, and a free copy of Ubuntu, while it's assumed your copy of XP (or Vista, presumably) is available and legitimate.
There are, of course, easier ways to get at multiple operating systems from one system, like using virtualisation software such as VirtualBox, VMWare, Parallels and many others. But if you're looking for the no-slowdown, full-on, multi-OS experience, Wired's got it laid out for you.
Triple Boot Mac OS, XP, and Linux on a Mac [Wired How-To Wiki]