Well i am gueesing you want a new processor and RAM?
Firstly, you need to check what RAM you motherboard will work with, i think you will be needing SDRAM, because it's an old computer right?
BUT, if you want a new proccessor, you WILL need to upgrade your motherboard, well, that's if it's a new proccessor (2.0+Ghz etc). And make sure that it's a motherboard compatible with the proccessor you are looking for.
You are better off to wait for Urban to reply, he knows what he is talking about lol. What i have put is just basics
EDIT: Also, when you purchase a new motherboard, you will need to find out what RAM is compatible with it. With the new Motherboard's out now, they can use either DDR or Sdram i think, well, most of them can :P