Core 2 Quad Q6700 vs Core 2 Quad Q9400?

Since you're overclocking, the Q6700 is more friendly to new people and weaker motherboard since it has a multiplier of 10. If you're an experience overclocker with a very good motherboard, get the Q9400, it has a better architecture, a unified cache, and 45nm to boot. The only downfall is that I believe that the multiplier is 8 and that it can't sustain a high vcore or high northbridge.