Your older system uses an unusual type of memory -
184-pin RIMM PC-800
This is RAMMBUS and not carried at many places - but still available. To determine what you get, you need to know how much memory you have now and how many modules are installed. So note your total RAM that's shown in the BIOS and then open the case and count the number of sticks in the 4 slots.
You can add one module at a time.
You can buy single 256MB modules, pairs of 512MB modules, single 1GB modules. Your system can support up to 4GBs of RAM.