It looks like it uses PC3200; it's hard to say without the exact model number.
Point your browser to the URL below and select the
Scan My System button. It will detect and suggest replacement modules for you.
Crucial Memory Advisor™ tool Once you know the module type you can peruse Newegg to compare prices (Crucial is good memory, but is typically more expensive than most).
Oops - Jenny beat me to it...