Upgrading RAM question

Ok so im a total noob in computers (i mean im seriosly a noob in computers). But iv been wanting to upgrade my RAM for a while so i can run my games better. But what i have heard is that not all RAM sticks can work for one system.

So how can you find out wich RAM sticks can work for your computer. (btw im wanting 1GB more RAM on my computer, and if anyone is asking, iv got a windows XP)

The quickest way without actually opening your computer would be to use a program called CPU-Z.

Go the "memory" and "SPD" tabs in the program and it should tell you exactly what kind of memory you have now.

http://www.cpuid.com/cpuz.php

BTW, RAM isn't the best upgrade if you are wanting your games to run better. It will really just help with load times and stuttering. The video card makes the biggest difference in how games run.