Well, often you have up the voltage to your CPU core or memory to get up in frequency, but I can tell you with my P5B-deluxe and E6400, I've gotten to 3.3GHz on stock cooling. Its "stable" but it gets to 80C under TAT load!! So I run it much lower to keep temps reasonable.
So, good ram is key, but even cheap ram rated for something higher than DDR533 should be able to do mild overclock easily. If you're really having trouble OC'ing, start with something really small like an FSB of 300MHz, then 333MHz.... Have you read the FAQs on this website?