Hard Locks in vista 64 bit

I recently built a new PC, and I've had this recurring problem whenever I use more than 2 gigs of ram. When logging into WoW, it starts loading up and at the end of the blue loading bar, it freezes. Also, if it manages to log into a character successfully, I can usually log in and out of that character just fine. Taking out one stick of ram makes the game run fine, but I'd prefer to use both in dual-channel mode to actually take advantage of more memory.

I've tried many tests, such as Memtest, Prime95, etc. and all work perfectly over extended periods of time without crashing. I even had a spare set of 1gig sticks and ran them in dual channel with no problems at all. Both Crysis and CoD4 both work perfectly with little slowdown, so the problem seems to be with WoW.

So far, I've tried messing around with the timings, voltage, moving the RAM to different DIMM slots, etc. I'm on my 3rd set of RAM and I'm not sure what the exact problem(s) could be. Only problems which I could think are 64 bit Vista, faulty motherboard, 3 very unlucky sets of RAM, some random software update, or maybe even some more Bios setting twinking. Any Suggestions are welcome.

Thanks in advance.

Computer Specs:
CPU: Q6600 G0
MoBo: EP45-UD3R Gigabyte
Memory: OCZ Reaper DDR2 PC2-8500 2X2GB (Currently Using), G.Skill DDR2 PC2-6400 2X2GB (Not Using), G.Skill DDR2 PC2-6400 2X1GB (Previously Used and worked perfectly)
HDD: Western Digital 500GB
PSU: PC Power and Cooling Silencer 610W
GPU: Nvidia 9800 GTX+
Stock Speeds.