Go to your local Apple store, they should be able to help you out. Hope this helps!
<b>Moderator Note</b>: Apple is generally the most expensive option for purchasing memory. Do a quick Web search, as most of the memory vendors provide tools such as scanners to help you determine the exact memory your system requires. You may wish to try <a href=”http://www.crucial.com/mac/index.aspx”>Crucial</a> and <a href=”http://eshop.macsales.com/shop/memory/”>MacSales</a>. Good luck.
from my experience i can help you out. here is your configuration for memory and processor but i think now its i obsolete product, you may no longer to get support from apple regarding your Mac. you can do it yourself,
Processor and memory
700MHz or 800MHz PowerPC G4 processor with Velocity Engine
256K on-chip level 2 cache at full processor speed
100MHz system bus
128MB or 256MB of SDRAM expandable to 1GB; one 168-pin DIMM and one open user-accessible SO-DIMM slot