Excessive memory usage only by PC games

Whenever I start my games COD4 MW and MW2, my memory usage for both games goes up into the 300,000 k and as I progress through the game, maybe 5 minutes, the game freezes in the 600,000 k range!
Is anyone familiar with what causes this? I didn't have these issues with my previous motherboard. I recently upgraded to a BioStar TA760G M2 which has on-board RealTek HD audio which I've read causes problems with Activion games but I've also installed non-Activision games with the same problem. I'm a novice builder, OC'er, and avid gamer. BTW, I did OC my machine, here are the specs;
nVidia 7950 GT OC'ed a bit
CorsairDominator 2x2gb 1600 DDR2
AMD 64 Athlon X2 4000+ 2.1 ghz OC'ed to 2.85 ghz
CoolerMaster 500w PSU
BioStar TA760G M2 mobo
Things I've tried;
Ran MemTest86 on two sets of DDR2 I have with no errors and tried both sets with same results.
Tried a non-Activision game thinking it was a problem with Activision programing and the components in my machine.
I have formatted and re-installed my games.
I have formatted and re-installed my video card.
I have formatted and re-installed audio drivers.
I don't get errors when I run the games, my Mem Usage just increases to a point where the game freezes.
Any ideas floating around out there??

I tried your idea, I disabled my manual OC on my motherboard but had the same results BTW, CPU usage is only around 15-20 when playing the games.
I am a noob when it comes to oc'ing, is there anything in BIOS I could try to change? I've messed around with all the memory configurations with the same results.

Ideas?