PC crash with reboot on CoD4, UT3, and CoH:OF

I've been having endless trouble with my new pc, crashing with no warning and immediately rebooting without an indication of the cause. It has the most trouble playing .bink videos for opening credits on CoD4 and UT 3, which makes it crash almost instantly. When I remove the movie folders for the offending games, they will load into the game, but after 10-30 minutes of play pretty much everything invariably ends in the game crashing with the same reboot. CoH-OF will play the intro videos, but also crashes with a reboot after about the same amount of time.

I've found that the .bink videos will play, and I only get in game crashing, if I use the display drivers that came with the GPU (version 7.8 catalyst drivers I believe), but with them up to date videos and in game crash like mentioned.

I bought the 2nd GPU (the ASUS one) thinking it was the sapphire card causing the problem, but on its own or in crossfire with the sapphire, the ASUS card still has the same crashing. I'm not sure what the suspect part is now. I've tried formatting, and even with just the display drivers and a patched copy of the offending games, it still crashes in all the same ways.

please, please, please help!

Here's my current setup:

MOBO:
Intel BOXDX38BT LGA 775 Intel X38 ATX DDR3 Intel Motherboard

RAM:
OCZ Gold Edition 2GB (2 x 1GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Dual Channel Kit Desktop Memory

CPU:
Intel Core 2 Duo E6750 Conroe 2.66GHz 4M shared L2 Cache LGA 775

GPU:
ASUS EAH3870 TOP/G/HTDI/512M/A Radeon HD 3870 512MB 256-bit GDDR4 PCI Express 2.0 x16
&
SAPPHIRE 100225L Radeon HD 3870 512MB 256-bit GDDR4 PCI Express 2.0 x16

HDD:
Western Digital Raptor WD1500ADFD 150GB 10000 RPM 16MB Cache SATA 1.5Gb/s Hard Drive

PSU:
Antec NeoPower 550 550W ATX12V Power Supply 100 - 240 V UL, CUL, TUV, CE, FCC, CCC, CB, C-tick


I admit I'm a newb at building gaming pc's, so any mention of some egregious error I've made would be appreciated...