AMD Phenom II X4 965

Ok I wasn't sure if I should put this in RAM or CPU so im asking in the general forum lol. I put my CPU in the title. The question is I just read through the BE OC guide on here and found that some of my issues may be coming from my mis matched RAM sticks. I have 1 4GB stick from Corsair and 1 2GB stick that came with a pre built I had. They are both 1333mhz but the CAS timing is different. Corsair is 9 the POS is 7 lol.

Sorry for the long explanation here is the main question. I play Crysis and use the in game info screen to see frame rates and such. It has a section that shows mem usage. It never goes over 1.5 GB and I have a 2Gb HD 6950 GPU. So does that mean that wile gaming im not using any system ram being that the total usage is less than what I have on my card. Also do you think I would be better off just using 1 4GB stick than using both mismatched and have the 6GB total. I also play Metro 2033 and that is the game I would worry about as far as cutting down my memory to 4GB.

So from what I can gather I would have better results OCing my CPU with 1 stick because I would not have issues with the ram not matching, but would those gains be wiped out due to having less ram.

Right now I have the 965 stable at 3.7 my OS will boot with it at 3.9 stock voltage. My 3Dmark 11 score at 3.7ghz is 4700ish and after that with every 100mhz clock bump my score goes down and not up. So basicly im looking at that as anything over 3.7ghz is not 100% stable.