New Computer. Which OS?

I just received all the parts for my new system. I'm building an Athlon 64 unit, and I'm trying to figure out if I should use my 32 bit version of Windows XP or try to get the new 64 bit version. I use my computer almost solely for gaming.

I'm also hoping that there are no driver issues. Anyone that has experience with this, please let me know what you've discovered. Here's my system specout:
AMD Athlon64 4000+
MSI K8N Diamond
2GB GSkill PC3200 Memory 2-3-2-5
2 Western Digital 74GB Raptors (RAID 0)
GeForce 7800GT 256MB
Sound is on-board Audigy2 SE

Thanks in advance.