Depends on the processor. A typical Conroe runs DDR2 at 266 MHz and thus requires DDR2 533 memory. Overclock the FSB and you need faster RAM. The E4xxx systems use a 200 MHz FSB and only need DDR2 400. If you get something more modern, the requirements go up. Overclocking raises the bar substantially. AMD AM2 systems work best with DDR2 800. Your mileage may vary...