If you have an open slot or two, pull the sticks out and match them. You'll probably find them on ebay or craigslist. Pricewatch also has alot of venders for ram.
Your INTEL 945 (GCPE or GCNL) will support only Maximum 2GB(2x1GB) of DDR2 RAM. You can get two of 1 GB RAM and install in the both slots for maximum Memory performance.