Visit Crucial Technolgy
http://crucial.com/uk/store/drammemory.aspx
They have a system scanner that will tell you what you have onboard and show you recommended memory and what your motherboard is capable of taking
Although it says mine can have up to 4 Gb and so does the handbook but when I added a 2Gb stick in one of the slots XP and post still showed it as 2Gb onboard when should have been 3
lol