I looked up your system at Crucial.com for memory, you can have up to 4gb of DDR2 memory up to PC2-6400 speed
So, you might consider getting one of these for 2GB (which should be enough) or 2 for 4GB

Newegg.com - Crucial Ballistix 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail

-OR-

Newegg.com - Kingston HyperX 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail