Don't know if this comes too late, but the E6600 is already a 3Ghz processor, therefore it MUST overclock higher than 3Ghz!
The E2200 is a 2.2Ghz processor and can be overclocked to 3Ghz! The 6600 (in MY opinion) is a better core 2 duo processor! Try overclocking it to about 3.5Ghz
Also BOTH are...