E8400 chances are you'll hit 4.0GHz x 2 cores (8 GHz)
Q6600 chances are 3.6GHz x 4 cores (14.4GHz)
Now with more and more games/apps being multithreaded, ask yourself this: How long do you plan to keep your proc before upgrading again?
IMHO, the Q6600 is more future-proof. Even if you only end up using 2 cores most of the time, the difference of 400MHz is minimal to none, especially in gaming.
Edited by Somenoob - 6/13/08 at 9:15pm