Ok, first thing, make sure you monitor your CPU temperatures closely when overclocking, "coretemp" is a good program to use. Make sure you keep you load temperatures below 70 degrees to be safe.
Does you PC have an aftermarket cooler or just the stock intel one? If you want to do some serious overclocking - £20-30 on a cooler is a good investment.
The best way to overclock is to find the limits of each of the parts (motherboard, RAM, CPU) . First find what the maxiumum FSB you can get is: do this by setting the cpu multiplier down to lowest (x6 i think) and slowly increase the FSB speed 10Mhz at a time and see if its stable using a program like OCCT or Orthos. Before you do this - set the RAM to run synchronously with the FSB (RAM usually runs much faster than FSB so this should be very easy on the RAM) as this makes the system more stable. Once you find the limit of motherboard, move over to the CPU.
Set the FSB to the limit you found earlier (maybe 10MHz less just to be safe) - then slowly increase the multiplier 1/2 a point at a time. Run the stress test after each increase to ensure its stable and the temperatures are safe. Keep doing this until it fails to boot, load into windows or complete the stress test. When this happens - you can do 1 of 2 things. 1) Stop there, put the multiplier back down to the highest stable level - do a long 6hr stress test to ensure it is cool and stable. Overclock complete. or 2) increase the volts until it is stable and continue the process until you reach worrying tempertures or voltages.
Throughout this - keep the RAM link synced (ie FSB-333MHz, DDR2 RAM 666Mhz ). There should be a setting for this in you motherboard. If you keep the FSB under 400MHz then your RAM will be under spec, so no worries. When you get over 400Mhz you will be overcloking your RAM - you may want to do a separate test on the RAM to find it's limit if you plan on overclocking it too. Best bet is to keep the FSB at 400MHZ (if you can reach it) and work from there.
I know this is a pretty big post, but its effectively a guide to clocking your chip (I have one of the q9550 chips too). If you have any questions or want me to clarify anything I have said please just ask
