Then you will be able to get that Q6600 to a good 3.2 to 3.4 GHz, depending on your cooling. And the overclocked Q6600 will destroy a Dual Core E8500.
I'm not so sure where you got this from - it's true that an OC'd Q6600 will match or beat (though not 'destroy') a stock E8500, but an OC'd E8500 will beat that - being as you can push it to 3.8-4.0GHz, something a Q6600 will never reach (at least on air). I'm talking about in gaming here, rather than multithreaded transcoding or something like that.
Additionally, it saves installing a new mobo as the E8500 will OC to 3.6-3.8GHz on the 650i mobo as several users here have done so already. Have a look at this page in bit-tech's recent lynnfield review: http://www.bit-tech.net/hardware/cpus/2009/09/08/intel-core-i5-and-i7-lynnfield-cpu-review/7
and (although they use an E8600) you'll see it beats the Q6600 at both stock and OC in gaming.
Having said all that, if you are confident swapping out your mobo it's probably not a bad idea as the Q6600 is a very good chip and the Dell 650i isn't brilliant - but if you really can't OC the Q6600 then the straight processor swap is an easier upgrade path, if not necessarily the most cost-effective.
Seriously lets get a idea of whats going on here and not base out stuff off crap online.
I can call a q6600 overclocked to 4.5ghz and a e8600 overclocked to 3.34ghz both overclocked. Then post bench marks of that q6600 moping the floor with that e8600 but is that fair? No
Encodeing. Dapending on the program most take use of more than 2 cores that Q6600 will spank that e8600 like a baby all day long. Both stock and overclocked.
Gameing. This dapens on a few facors.
Game. Was it made to use more than 2 cores?
Graphic card. Example a Geforce 8800GT needs a core 2 duo or quad at about 3.2ghz to unbottleneck the card.
So in coveing those the first problem the Q6600 has that the E8600 does not is the fact it runs at 2.4ghz. A simple 1333 buss overclocked helps by overclocking it to 2.99ghz and this is a close most q6600's can hit very easy. But you will still wanna oc to 3.2-3.4 to releave the bottleneck 3.2 is prefered.
If the game can take advantage over more than 2 cores then the choice is simple get the Q6600.
So you you need to project into the future alittle if you wanna base your choice on gameing alone. The choice is easy go for the quad. Tech is growing every day and game devlopers are takeing advantage of tech. If you plan on upgradeing to say a core i7 or smthing higher in the near future and most of the games you play only use 2 cores then the easy choice is go with the dual and ride that for as long as you can.
Another factor to consider is heat and power useage. The quad will take power power to run and produce more heat but not double, close to it but not.
Normal every day use. internet and stuff like that most people say a quad will not make a diff. Well im going to itt tech and i decided to challange that clam. So i did a little experment
4 test systems were made. Each had the same specs and 2 had Q6600's and the outher 2 had E4600's. Each system used the same exact parts at the same settings and ran windows vista hom prem 64 bit. After a test subject was given 30 min of timed use on each machine the harddrive was switched to a pre imaged fresh harddrive and the next subject went on that machine. The users were able to id the quad core computers an said they were more responsive and felt faster. I can confirm this by also doing the test but i am not counting my self as i knew what whs in the system.
Also not the test subjects were watched to enshure that they did not check anything that would reveal the type of cpu in the system.
So the only person who can awnser your quest is you. If you read this and awnser the questions the choice will be very easy for you.
I leave you with this. The Q6600 is still a beast to this day dispite how old it is. That cpu wll be part of the cpu history as a quad that was never expensive but offered amazeing preformance
But to awnser your question. The cpu will drop right in and work. But if your Q6600 wont oc dont expect the E8600 to oc since the stock mobo in the 630i dosent like high fsb's and going from the 1066 fsb to the 1333 will hurt your overall ocn. Also when you oc you want the lowest fsb chip with the highest multi.
I think your q6600 wont oc because of one of the fallowing,
You memory is beaing oc'ed to much. Unlink your memory remember when you oc you will need to lower the memory davider ir you will be also overclocking the memory.
Disable ce1 and speed step
Set your core voltage to somthing close to yoru vid. 1.35volts for a q6600 is pretty nice and that was one of the higher vid's for the chip. Start with your voltage there and if your memory is set up right you will hit 3ghz with that chip no problem
LOCK YOUR PCIE BUSS!!! Lock yoru pcie and pci buss. PCIE should be about 100mhz if you dont do this it can damage your pc and cause stability problems. I prefered a overclocked pcie buss or benchmarking but thats a little more advanced that just overclocking yoru cpu.