I need help on choosing an OS. Right now I could use (and have access to) Vista Home Premium 32 bit, or XP pro 32 bit.

First which of these two would be better(and why)?

Also if I could get my hands on either 64 bit, do you think that would be better (I have heard that 32 bit programs such as most games run better in 32 bit OS)?

here is the new rig

CASE: Antec Nine Hundred (already purchased)
CPU: Intel® Core™ 2 Duo E8400 (already purchased)
DVD±RW : Samsung SH-S203B/BEEN 20X SATA Burner w/Software
FLOPPY: Sony 1.44 MB FLOPPY DRIVE
FAN: Zalman CNP9500 LED CPU Cooler (already purchased)
HARD DRIVES: RAID-0 with 2 150GB WD Raptor 10,000RPM SATA150 16MB Cache WD1500ADFD
MOTHERBOARD: EVGA 132-CK-NF78-A1 780i chipset
MEMORY: 4GB (4x1GB) PC6400 DDR2/800 Dual Channel Memory Corsair XMS2 CAS 4-4-4-12
POWERSUPPLY: Antec NeoHE 550 High Efficiency Power Supply
VIDEO: EVGA 512-P-N841-AR GeForce 8800GTS (G92)

I'd say if your not paying for it, try out Vista (or Vista64) maybe borrow the DVD from someone to see if it works for you (just don't get your hopes up.) That way if it doesn't work no harm no foul.