From stock core and memory clock speeds of 500MHz, I was able to overclock the Sapphire Radeon X800XT PCIe's core 46MHz and memory 55MHz, yielding a final clock speed of 546MHz/555MHz (core/memory). It's important to note that the core speed is actually faster than the stock core speed of the Radeon X800XT Platinum Edition, and just 5MHz slower than it's memory clock speed (Core: 520MHz / Memory: 560MHz). I was impressed that we were able to get at and maintain these speeds throughout our testing, with no artifacting or problems at overclocked frequencies after more than a week of playing games at them.