The newest batches of Q6600s can't overclock well, needing a lot of voltage juice to get decent clocks, I hear.. I would choose a Q6600 over an E8400 if it were a matter of clock for clock and the Q6600 can hit 3.6GHz stable. But now I dunno
Many people argue that games don't utilize quad-core tech, but I don't think that's the point really.. Most games I've played have never been limited by my Q6600 (overclocked to 3.2 for daily use) what I have found is that alt-tabbing something like Crysis to a 10+ tab session in Firefox and listening to a massive music playlist on winamp, while unpacking a large .rar file, whilst encoding a bunch of videos to play on my mp3 player is much more manageable on a q6600 than on my friends' E8500 rig running @ 4.0GHz