The max amount of RAM that a computer can have is determined by the operating system (windows) and the motherboard. Windows XP is said to have a max amount of RAM of 2.5 gigabytes. A motherboard will limit the RAM to 8 gigabytes, 16 gigabytes, or something like that. Check with the operating system and the motherboard to see how many gigabytes of memory the computer can use.
If you use windows vista or windows 7, you can have much more memory at your disposal. Windows Vista naturally takes more memory than Windows XP and does not cap at 2.5 gigabytes.