Overclocking this chip has been a pure pleasure. When paired with the X38 chipset, the potential could be far out there with the correct cooling setup. I started at 400 x 9 and 1.35v and the system booted so flawlessly that I knew that it was thirsty for more. So I jumped the multiplier to 9.5 and kept raising the FSB by 15MHz while keeping the voltage the same until it would not boot. Then I upped the voltage and started again. In the end I was able to push the Intel Core 2 Duo E8500 chip to 4.322GHz before the temps got high enough to make me not want to take the chance. I ended up with the E8500 at 455FSB x 9.5 multiplier for the speed I obtained with a voltage increase to 1.45v. At 4.322GHz I was able to complete SuperPI, Orthos Prime and all of the benchmarks with no problems at all.

http://www.overclockersclub.com/reviews/intel_e8500/