You got 2 sticks of 256, in dual channel mode, which gives a free boost in bandwidth, meaning its a bit faster.
You need atleast PC2700 ram, and your latencies are pretty much generic, so get any DDR stick of 512.
How many DIMM slots does your Mobo have? If it has 4, than you can keep the dual channel, if it has 3, than you might lose it.
edit: (bloody slow typing

) On CPU-Z click on mainboard, another print screen of it pleese

)