You want max OC or just a bit faster? Since you don't mention what version of the P5Q, i'll be generic.
Be 100% sure your ram is up to the task, set it to 1.4vcore, 1.4v FSB Term and whatever voltage your ram needs to hit ~500fsb. Crank it to 8x500, set your ram timings to 5-5-5-18 or looser (depends on ram), the ram divider to 1:1 and tweak your way from there. If it won't do 500, leave those voltage settings alone and change the FSB to 451. Just keep messing with it until you get it running. Then after you get into windows, start stability testing. If it passes stability, keep lowering the FSB Term and Vcore until it crashes.
There are many more tweaks but that should be plenty to get you started at least.
FWIW, I know the E8xxx cpus will run well. I've had mine running 545fsb before I think my ram started to crap out. The 45nm chips are a pain to OC though as they are VERY voltage picky. One setting off and it won't load windows.