I've been toying with changing my Q6600 for an i3 setup of some sort, mainly for the lower power requirements, but I keep stalling! I have no heat issues as my PC is watercooled.
For those who say their system is 'snappier', how much of this is down to the change of mobo and memory as well? I must admit that I have no complaints on the responsiveness of my current setup, so I guess that's why I'm hanging back.
It seems like I would be spending a fair chunk of cash, along with the hassle of selling off the old components and the upgrade/reinstall process, to save a few quid a year in electricity bills. If it was going to be a lot faster for my uses (photo processing/browsing/gaming) then maybe I could be more tempted...
The i3 does look like a very good CPU though, the one to buy out of the range for sure. It's incredible how much the intel dual cores have come on in recent years.
TBH if you are happy with your PC, and you have no issues with it then it would be a waste of cash to move to an i3 from a Q6600.
Speed wise I had no complaints about my Q9550, it was a great CPU and a great setup, I only changed because I wanted my PC quieter and smaller.
The 'snappyness' is there, but its nothing major, nor is it worth shelling out and messing about selling an entire rig for just to attain. You can honestly live without it, its just small things like when you open a window its instant, whereas on the 775 boards it always felt like it had a pause before it appeared, alt tabbing (in/out of games) seems faster as well, how much of it is placebo effect, and how much is real I don't know, but all I can say is i3 feels more responsive in a windows environment than my Q9550 did.
HOWEVER, do not go selling your rig just for that, the Q6600 is still an awesome CPU, and once overclocked more than capable of running and playing anything out there.
The only reasons to change from a 775 quad to an i3 are heat and powerdraw.

