Hi, there's a very easy solution to that. What you need to do is download, install and run PC Wizard:

www.cpuid.com/pcwizard

Then once the window is open, look for "phsical memory".This will tell you what kind of RAM your motherboard uses eg DDR-SDRAM PC2100,PC100 etc. So when you're buying new RAM you need to simply make sure it is the same type then get the size you need.
Also, your motherboard will have a limit ont he amount of RAM it can hold. That can also be found using PC Wizard.

If you need any more help, just post back.