The ram for your system can be found at the following
www.intel.com/products/motherboard/D945GCL/ According to them you can use either DDR2/677 mhz or DDR2/533 mhz also refered to as FSD{front side bus speed}
I hope this helps you out, you can get ram at bestbuy, or any other computer store. Shop around E-bay has some very good deals on brand name new memory.
Good luck,
Troy