as far as i have noticed, theres very little difference between the two, a 3Ghz Q6600 will be almost impossible to tell apart from a 3Ghz i5 for example. wouldn't bother upgrading from Q6600 for i3, i5 or i7.
I've not got an i3/i5 etc but from what I can see heat output/power draw is lower on the newer CPUs, that along with performance increase in things like rendering etc might make it a worthwhile upgrade?