at stock, on most apps and games u may see a small drop in speed and FPS, due to the slower core spead of the Q6600,
in apps and games that utilize the full capability of the quad, u will see a gain in framerates and speeds.
even tho the Q6600 is a great CPU, i personally would recommend the Q9550 or even the Q9650 (as the prices have dropped).
the chips are only between £50-£70UKS difference to the Q6600, and will give u a nice little boost all round the spectrum (software apps and games)
another thought would be to get the Q6600 and overclock, i know u said u wont be overclockin, but when u can get the Q6600 G0 steppin from 2.4 to 3.2 without changing the V-core on most chips, this only would add roughly 3-4c temp difference from the stock speeds, and give u a substantial speed increase for ur apps and games.
although if u are gonna consider an overclock, i do recommmend an aftermarket cooler.