or just go with one stick of 512 mb memory. Dual Channel mode is nice, but most likely you won't notice much performance increase between 512 memory single channel or 512 memory dual channel via 2x 256mb memory sticks unless you're doing heavy duty memory gobbling things on your pc.
But if I were you, if I could afford a new PC, I'd go get it rather than trying to upgrade a 7 year old PC.