Practice Auto 3,2,1,
If it is a Compaq, Dell, HP, Gateway, IBM, etc. it is easy to find out as TCS said. (If you like, you could give us the make & model, and we would look it up for you.)
If you had someone build the computer for you locally, we just need the motherboard info that you see on bootup and we can find out the type of RAM it uses too. (If it was built locally, they should have given you the motherboard manual, which tells you which RAM the motherboard uses.)
If you are going to run WinXP and you are not planning on doing Audio/Video/Graphic editing, you do not need more than 512Mb of RAM. I hate seeing people drop a small fortune on 2Gb of RAM, when they could save that money or even use it on DVD Burners, or new Vid Cards, etc. Something that will make a difference in the performance of their computer.
Take Care,
Richard
P.S. Unlike what the Memory Makers tell you, too much RAM actually slows a computer down.