You have an older motherboard. It only supports PC2700 (333MHz). More modern standards are PC3200 (400MHz) or even faster.
Okay, having said that, I'm no expert but here's what I've read about overclocking and RAM:
The higher quality RAM, or the stuff advertised as good for overclocking, will accept higher voltages without shorting out and will run at faster speeds. Overclocking changes the frequency the RAM runs at, as well as the voltage running through it (sometimes, depends on the options you choose in the BIOS). So you're going to have to go with premium stuff, unfortunately. This means more $$$. As to the specific types, I can't list a brand or anything, I don't overclock.
EDIT: Note that overclocking can have other side effects. It screws my joystick over when I try it (screws with the gameport clock frequency, or something).