Long story short, the ram and cpu FSB business is a bit... unfair.. to unsuspecting buyers.
Essentially intel multiply their actual fsb by 4 so at 1066 MHz it's really only 266Mhz and DDR2 RAM's actuly fsb is multiplied by 2. So PC6400 is 400MHz (far more than you really need if not overclock) and PC8500 is really 553MHz (rediculously more than you need)
PC6400 is a good match for 1066MHz FSB chips (like the Q6600 and Q6700 because it allows you to easily overclock the CPU to 3.6GHz + )
With regard to overclocking, as long as you stick to the rated voltages (stock volts) you don't invalidate the warranty.
You will almost certainly be able to hit 3.0GHz on a q6600 without touching the voltage and the tempreatures (which you want to keep below 60 degree's ideally under full load) should remain nice and low as long as you buy a half decent aftermarket cooler.
If you just wanted to get a q6600 to Q6700 speeds though it would litterally take about a minute of your time and would barely change the tempreatures of the chip, theres a great overclocking guide in the stickys of this forum by
clunk