There are several people inquiring about how to overclock their Q6600's right now, so I though we could join all of those discussions into one big thread so that everyone could benefit, and also make it easier on those of us giving the help.
If you have a Q9000 series cpu, you will NOT be able to overclock using the stock motherboard, sorry. This guide is intended for all E8400/8500 and Q6600 users primarily. If you have a Q9k series cpu and want to overclock, you are going to need to look into a motherboard upgrade, which there is a sticky thread already posted giving some tips by drBlahMan.
First, and foremost, before you start overclocking anything, take time out to first make sure that all of your fans are running properly, and get rid of all that dust you have collecting in the cpu heatsink. Overclocking creates heat, and having dust bunnies flying around inside your case won't help. So, buy some compressed air or use your dad's air compressor with a blow gun attachment and blow all that dust out. DO NOT use a vacuum, or brush of any kind unless you want a good reason to upgrade your motherboard.
Once it's all cleaned up, we need to make sure your system can pass a stress test with all stock settings. If you have already tried overclocking your rig with the NVidia software, or in the BIOS, change all of those settings BACK TO STOCK. We are starting from scratch here, so put everything back to stock, and turn off any power management controls you have turned on like auto sleep, hibernate, hard disk shut down, etc. We want everything to run at 100% all the time from here on out. You can change this later, but while we try to get you to a stable overclock, these all need to be turned off.
At this point, you should have everything free from dust, visually confirmed that all fans are fully operable (especially the cpu heatsink), and all settings are back to stock just like the day your rig was born. If you haven't updated your BIOS in a long time, go ahead and check to make sure you have the latest revision from the Dell site, and get that installed.
Now, let's stress test your stock system. Your system should pass this test with flying colors, and with low cpu core temps (more on this in a moment). The test is going to do two things. First, it will tell us if you have another issue that needs to be addressed prior to overclocking, like a bad memory module, or defective motherboard. Second, it will tell us if your cooling solution is in good working order. If your cpu core temps are high during this test, at all stock settings, then something is wrong and it needs to be addressed prior to overclocking. You may possibly need to remove your heatsink and reinstall it using a better thermal compound like Arctic Silver 5, or something along those lines. Most people will have no issues running this test, but I have seen some run into major heat issues that needed to be corrected first.
The program I like to use for monitoring cpu core temps is RealTemp. Download it here:http://www.techpowerup.com/realtemp/
It's a very simple program that will monitor your cpu core temps, do not make any changes to its settings, and leave it running while you run the stress test. It will record your minimum and maximum temps, which will be what you will post back with later.
Now, on to the actual stress test itself. The test of choice at the moment is going to be OCCT. Download it here:http://downloads.guru3d.com/OCCT-%28OverClock-Checking-Tool%29-3.0.0-download-1880.html
Before you run this program, be sure all other programs (except RealTemp) are closed. Double check again that all power management has been turned off, and crank up your fans via the Nvidia Control Panel. Set your cpu and front fans to 100%. Yes, it will sound like a jet engine, but there's nothing we can do about that.
Once you have RealTemp and OCCT open, with all other programs closed, and all your fans screaming, go ahead and run the "CPU LINPACK" test being sure to check the box next to "Maximum 90% Free Memory" and "64bit" (if you are running 64bit Windows) for the full default 1 hour. Don't use your computer during this test. Just let it do its test and go play HALO in the other room or do something to occupy yourself for one hour. Don't worry, if your temps get too high (80C or above), the test will automatically stop and alert you of a problem. Same thing goes for any other issues. If it detects any errors, the test will stop automatically.
.......Fast foward one hour........
You should see a message saying you passed the test, and also some graphs showing the results. You can save these graphs if you want, or you can delete them, up to you. Assuming you passed the test, write down the min. and max. core temps (two for dual core and four for quad core) from RealTemp, because I need to know this before we can overclock.
If you DID NOT PASS THE TEST, you have some issues that need to be handled first. Tell me what the error message is and we will see what we can do to fix it.
Congratulations, you just stress tested your system. Get use to doing this because you will do it MANY times from here on out.
This was only one test, there are many other stress tests that we use, but more on that later.
Now, something you need to know before we make any changes in the BIOS. DISCLAIMER:
ANY CHANGES YOU MAKE TO YOUR BIOS IS DONE AT YOUR OWN RISK. IF YOU ARE NOT WILLING TO ASSUME RESPONSIBILITY FOR MAKING SUCH CHANGES, STOP NOW!!!
Okay, something else you need to know. The NVidia software doesn't like it when you make changes to your BIOS. Actually, its NVidia "Performance" that doesn't like it. Performance takes a snapshot of your system when its first installed, and if you try to make changes to the BIOS, Performance will just undo those changes when Windows starts up. So, from here on out, any time you want to make a change in BIOS, you will need to go into Control Panel - Add/Remove Programs, and remove the Nvidia Performance software first. This is also the software that controls your fans, so after you remove it, then go into BIOS to make your changes, you will want to immediately reinstall it so you can turn your fans back up.
Download it here:http://www.nvidia.com/object/nvidia_system_tools_6.05.html
I just save it to the Desktop so it's easily found. You will uninstall/reinstall this program a LOT, so get comfortable with it........unless you install a manual fan controller which would allow you to control your fans with ease, and delete the NVidia Performance software permanently, but this is another topic for another thread.
Now that you have your idle and loaded cpu core temps recorded, I need to know more about your specific system. This is where each individual overclock is going to be different, so from this point, each case will be handled individually. No two overclocks are the same since there are so many varying factors. Don't get upset if someone with the same hardware achieves a higher oc than you, its the nature of the beast.
Anyway, here's a BIOS template. Fill it out in its grand entirety, and post it back up with all of your info, along with your recorded cpu core temps. Somone will respond to you with their suggestions.
CPU: **** & Stock Clock (****GHz)
RAM used: **** (example: OCZ Blades 2 x 2GB's @ 1066MHz)
CPU Heatsink: Dell OEM
Power Supply: Dell 650w
BIOS Version: 1.0.*
Additional fans: (if other than stock)
CPU Idle Core Temps:
CPU Loaded Core Temps:
CPU Freq, MHz: * GHZ
FSB Reference Clock, MHz: **** (This is your *stock* FSB (QDR), MHz)
CPU Multiplier: [*X]
PCle Bus, Slot 1, MHz: 
PCle Bus, Slot 2, MHz: 
**Overclock FSB & Memorv Config**
FSB - Memory Clock Mode: [unlinked]
FSB - Memory Ratio:
FSB (QDR), MHz: [****]
Actual FSB (QDR), MHz: ****
x MEM (DDR), MHz: ****
Actual MEM (DDR), MHz: ****
**Memory Timing Setting**
Memory Timing Setting: [manual]
tCL (CAS Latency): *
Command Per Clock: [*T]
** Advanced Memory Settings **
CPU Core: [*]
CPU FSB: [*]
Chipset Voltage nForce SPP: [*]
C1E Function: [Disabled]
Execute Disable Bit: [Enabled]
Frequency Unlimited: [Enabled]