you can stick 1.25GB in there, which includes the built in 256, so that means a 1GB stick. People recommend crucial, but i'd go to newegg and look at what's cheap. the kingmax ram that's the cheapest has several reviews saying it works in their mac, so perhaps that. http://www.newegg.com/Product/Product.asp?Item=N82E16820155110
HJW is correct.
I will respectfully dissent on the recommendation though.
I say get your RAM from a vendor who tests and guarantees the RAM in your model Mac, offers a lifetime warranty and a no-charge return if it doesn't work. (Which would not be one of the PC Generic discount places)
Some members like Crucial (despite their generally higher prices), OWC, RamJet. If you are in the USA I like Data Memory Systems www.datamem.com