Noob RAM Question

[TellyDSP]

My PC is fairly old, and I'm too cheap to buy my laptop yet(just graduated, and need a job first).
Question is...I want to increase my RAM, and I'm currently running PC100. Do I have to use PC100, or can I use any speed? Will I get an advantage/disadvantage?
From what I think I know, I can't use different speed RAM, so if I were to use non-PC100, I'd have to ditch the ones I have now. I'm just curious if it's worth going, and/or necessary, to use something other than PC100.

Tried finding it myself, but couldn't quite find what I was looking for.
TIA
Craig

[extreme]

You can use PC133 along with PC100. Anything higher is backward compatible, but not vice-versa. You can not use DDR SODIMM memory on a system with PC100 or PC133.

Look for the best prices at: www.pricewatch.com.