Upgrade RAM on 2008 pre-unibody macbook pro?

What is the best way for me to go about upgrading the RAM on my 2008 macbook pro 2.4 GHz Intel core 2 duo with 2GB DDR2 SDRAM. The RAM is in 2 1GB cards

How high can I go? Do I need to replace both, or could I buy a 3GB and simply replace one? What kind of RAM should I get, and where should I get it from?

I would like to have atleast 4GB, but more would be better.