Need some pro help

my system spec are
Q6600 GO +thermaltake extreme 120 hs
Crucial Ballistix tracer 1x1 2gb 8500
8800gt
GA P35 DS3P
750 WATT psu

Now problem is when i try to overclock my q6600 from 266 to 333 on bios option without changing the any voltages with dual channel ram it doesn't boot up at all had to reset cmos like 30 time so far no luck. I took out of my one of the ram stick from mobo used single stick to overclock from 266 to 333 it boot up fine then i though maybe i have bad ram? so i swap out my other stick and ran on single stick to see if this stick is defective but it ran fine without any problem. so i can't seem overclock my q6600 with 2 sticks for some reason. any ideas ?
and I even ran memtest86 for almost 8 hour no error found.