I'd suggest you check the motherboard specs first, to see what the maximum amount and speed of RAM that it will accept is.
You can either do it the old fashioned way by taking off the side/top panel and visually inspecting it for a manufacturer and model number, or, use Belarc advisor
Run the advisor and look under the Main Circuit Board Heading. Go to the manufacturers site and locate the model number. Sometimes this will be only available in the support section if the board's a little out dated.
Either check the specifications or download the manual to find out what RAM it'll accept. look for maximum amount and maximum speed - the highest DDR speed.
If the board will take say DDR2 800, which may be listed as PC6400 RAM (800 X 8), then I'd suggest replacing the existing memory also with a matched pair of supported memory to improve on what you already have. The board will only run the RAM at the speed of the slowest memory installed.
If you find that the maximum is anything lower than PC6400, then yu may want to ask you local PC store if you can "try before you buy" a stick of DDR2 800/PC6400 RAM, and add it into the second slot. RAM is usually backwards compatible with slower speeds, so you may find that the board will run it.