Providing your computer can take it you can install what memory you like 128mb, 256mb, 512mb or more depending on the motherboard. It is the motherboard together with the speed of your processor (cpu) which decides what memory you can have and how much of it.
Go to here click here if you don't know the make of your PC then do the "Scan Your System" thing. It will tell you how much memory you have, how many memory slots are free and how much more memory you can add.
Thanks for your responses. Yes he does play games on it. When I have tried to reserch this, the articles seem to assume that the modules will be matched. They talk of the memory only running as fast as the slowest module. Will adding 256 mb RAM make any appreciable improvement do you think?
The Crucial site is fine if it lists you pc, it does not list my Advent 3618! doing a scan showed mine to have 1 x 256MB Ram and 1 x 512 MB RAM with a 3rd slot free! I have 1 x 1GB and 1 x 512 MB in two slots with no free slots left!