I've noticed there are various different speeds in ddr memory. But how much of a performance difference would I notice between them? Do I really need 400mhz memory, or would I only notice a minimal increase like with dual channel memory?

Thanks

Athlon XPs aren't that sensitive to bus speed, but you want to get RAM that matches the FSB you intend to run your processor. If it's a 400 MHz FSB Athlon XP, you need DDR400 memory to match it.

You can run your RAM at a different speed to your FSB, but the latency penalties are relatively significant. If you get a 333 MHz FSB Athlon XP, it's better to run your RAM at 333 MHz (a 1:1 ratio) than to run your RAM at 400 MHz.

Only get a higher rated DDR if you're going to overclock.
But as Spod said, always try to match your CPU's FSB.

For example I have a 1700+ that's stock 133MHz FSB, but I'm overclocking it with a FSB of 166MHz, so I bought DDR333 to match that FSB.