What should I do in order to dual boot XP on my Vista machine?

Sup folks. I have:
680i nVidia Motherboard
8800GTS EVGA GPU
Corsair XMS2 DDR2 6400 something-something RAM
e6600 CPU liquid cooooooooooooooooooooooooooled
Seagate 7200.10 320 GB Hard Drive
Windows Vista Home Premium 32-bit

Despite this, I know little about computers outside of using popular software, etc.

I want to dual-boot this machine with Windows XP, and the reason is: Vista is making Halo:Combat Evolved (my favorite online game) very slow at 20fps.

This may be nVidia's fault, but people who use XP and have the same (or closely similar) hardware and drivers as me don't have this problem.

I have 50GB of free hard drive space. Windows Vista is my current OS.

I love Halo so much I am willing to drop some more cash into my computer to dual boot with Vista and XP so I can play it s'more.

- What would be the best way to do this without spending a lot of money?
(I don't have XP lying around to use, I will need to buy it before it gets phased out by Vista.)

- Can I dual-boot when I have Vista already installed on my computer? And with my personal and program data being still on it?

- What would be the absolute best way to dual-boot?
(I know there are several options but I want to choose the most reliable path with no ill side-effects.)

- Should I buy a smaller/less expensive hard drive and install XP on it? What would be the simplest/easiest way to go about it if I were to?

I'm really paranoid but I want to do this. So I can continue to pwn at Halo.