Technically, you can use any speed RAM in your motherboard so long as it's DDR and 184 pin. You can range from PC2100 to PC5000 and things <should> work. (note: not all motherboards support the old PC2100 or PC2600 speeds anymore, but most do)
When overclocking, it's best to get RAM that faster than PC3200/DDR400 or faster than what the CPU's FSB is rated at. Why? The most common and effective way to overclock your system is to increase the front side bus. When you do this, the RAM speed is automatically bumped up.
If you have DDR400 (PC3200) RAM and OC via the FSB, you're going to hit limitations. PC4000 is rated at 250mhz (500mhz DDR) while PC3200 is rated at 200mhz (400mhz DDR). Hence, the limitations of overclocking are much higher.