As you increase the system clock above 133 MHz, your memory speed increases also.
At some point when your memory goes beyond spec, you must set it manually (in BIOS) to the next lower speed.
Then as you OC, the mem speed will increase, but not beyond spec.
If you OC enough - you can get the memory back to its rated speed again! (Heheh... heckuva OC.)
Point is, I believe this is what you are encountering. Your memory will no longer POST...
I have an i7-920 and trust me, you don't need to OC!
With my Gigabyte mobo, the 920 runs with a 22x multiplier all the time anyway and gives me 2.933 GHz.
I have better results disabling hyperthreading for playing games like Crysis...
Most games are usually much happier with 4 real cores, not 8 logical ones.