throbb: You should be able to get to 2.8ghz without touching the vcore. I was running mine at 3.2ghz on stock cooling.
Azzura: Manually set Vcore to stock instead of auto. Disable C1E and Speedstep. Lock PCI to 100MHz or is it pci-e (sorry been awhile since oc'n my q6600...either or it won't hurt to lock pci-e or pci).
Set the DRAM ratio to 1:1 while overclocking
Increment FSB by a small amount, leave multiplier at 9. Run Prime95 for 3 or 4 hours. 20 min should give you a relative idea about the stability, do the 3-4 or 24 hrs test once you get to the clocking you want.
If stable, increase FSB by another small amount. If unstable, increase Vcore by a step.
Continue until you reach target, or until temps/Vcore rise to high for liking.
I had mine running at 3.6GHz (been years now since running a q6600). OC'n with top of the line HS/fan for the day, reached 4.0Ghz on air with some creative big house fan help. 4.0 GHz was much easier to do with phase change cooling though.
Q6600 will easily/should do the 2.8GHz pretty easily. Possibly it won't take much of v bump. Just keep an eye on temps if running stock hs/fan.
Azzura: It has been a longtime since I OC'd my q6600. All my notes are either gone or in a book somewhere.
What I think I probably had to do was run loser timings on the ram, change the ratio (pretty sure it was 1:1...or as low as you can go) and most likely had to bump the voltage up on the ram.
I don't suggest you do any what I said above until you do some research on it.
I barely got 4.0GHz on air...the chip just runs too hot for air (it was just booting at 4.0 into windows, I could not even bench at it). Water might/probably would be able to handle it...but it would most likely require a pretty jazzed up big water cooling loop.
But water cooling isn't something I have done...I am pretty certain those self contained water cooling cpu units wouldn't be up to the task of a q6600 at 4.0GHz.
I say the line above because----
The phase change cooled the cpu down to -44C at idle, but once I started benching....even it still struggled to cool the cpu running at 100% during benching. When things are at -44C...I didn't really worry about what the Voltage setting was.....that and I had my 920's by then.
3.2 GHz is a good safe overclock (not likely things are going to burn out)...3.6GHz is doable for 24/7 operation but still borderline risky, even if temps ok (most likely the cpu temps will be bordering max recommended temps). 4.0Ghz....you might get into windows and do a quick SuperPi bench...but I doubt your going to be running the computer 24/7 at it (if that is your goal).
If you are going to follow through and play with pushing it -----Google it - overclocking Asus P5K SE q6600 and pay particular attention the the overclockers.net forums (if you haven't been there already).
Something I found on an overclockers forum -
"Yeah, seriously I have a high end water system(thermochill 120.2 w/4 fans and a dtek fusion) with my quad and I have a hard time keeping it under 60c under full load at 3.6ghz using 1.5v"