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The Q6600 is only a 266MHz FSB part, so don't expect miracles while increasing the FSB. With 400MHz it was possible to boot into Windows, but stable operation was out of the question. Due to the fact you can't increase the CPU multiplayer you are stuck with the highest FSB you can get. In our case, we got 3420Mhz with FSB 380MHz, but it was not very stable.
For our benchmarks we reduced the FSB to 370 and got a nice 3330MHz which was stable in Prime test all the time. The Q6600 can take more voltage than its QX6850 counterpart, at least our samples did. VCore at 1.5000V did not cause any problems.