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.
Don't know where your from but in the US a Q8300 can be had for 189.99 and the Q8200 for 169.99. They are both 45nm meaning they'll run cooler and consume less power than the Q6600. Q8200 and Q6600 are almost identical in terms of performance and the Q8300 is a little faster.