Ram or Mobo prob?

Hi all, my friend just helped me build a new computer mostly to be used for photoshop. And I'm running into a problem----

After installing the OS (XP pro SP2) and all the drivers/etc. I noticed the system was pretty unstable. The things I noticed were - programs unexpectedly quitting, occasional blue screens, and all my downloads seemed to be getting corrupted. It also almost always crashed when running BurnInTest (the program not the comp). I did some web searches and it seemed like checking the RAM was a good idea. So I checked the ram with the Windows Ram Test and each time around it had 125+ failures on the LRAND test.

Next I tested each stick by itself with the Ram test, and they all passed (5 passes each). Then I put in two dimms and tested, they passed. 3 gigs passed as well. Only with with the full 4 gigs does the mem test fail.

I am currently running two gigs and the system seems perfectly stable, all the above mentioned problems are gone and the comp passes the full BurnInTest every time.

I'm not super experience with computers and I'm pretty stumped on how to get my full 4 gigs running..

System details -

ASUS P5W DH DELUXE/WIFI-AP LGA 775 Intel 975X ATX Intel Motherboard
MSI NX8600GT-T2D256EZ GeForce 8600GT 256MB
Thermaltake W0101RU ATX 12V 2.0 Version 550W Power Supply
Intel Core 2 Duo E6750 Conroe 2.66GHz LGA 775 Processor
2x CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 675 (PC2 5400) Dual Channel

Any help you guys can be would be greatly appreciated

p.s. I know Xp can't use a full 4 gigs of ram but thats the only way to get it running in dual-channel with more than 2.

Well I popped all 4 gigs in and set the voltage to the Mfg. specs and it passed the ram test twice and the burnintest. Seems like that fixed it.

Thanks alot!