Allright, I can see a few things that may be your issue.
The first would be the value Mb. When you buy cheap you can not always expect top OC's. If you have a mesh on the side of your case, try mounting a fan to it blowing on the north bridge and voltage regulators (the little cubes surrounding the CPU). This should help cool your system down a little and may allow you to go a little further.
With shatty RAM it is probably DDR2 666??? If so, you should consider your self lucky to have gotten even a minor OC out of the major value lines. DDR2 is cheap as dirt right now. You should really get a matched kit like this
Corsair 4Gb DDR2 800 kit for $66.50. That will give you better performance than your current RAM that is not running in duel channel and also give you OC headroom up to
at least 400Mhz FSB.
Your temps are still very good. Make sure your BIOS is the latest then start to play with the voltage. Bump it to 1.55V and see where that will take you. Stress test with
prime95 in windows while keeping an eye on your temps with
coretemp.
Also, what revision is your Q6600? G0 or B3?
Start here and let us know how it goes.