First off we need to know your memory configuration.. what is in it now? Is it two 512MB sticks? or 1GB stick? How many Ram slots do you have?
Also you're going to find out what motherboard you have and look up the specs to see what kind of memory it can handle.
The IDEAL way to do it is to have all identical sticks at once, but it's not absolutely necessary, nor is it always possible, or affordable.
Good luck.