If you can find them you might as well go for a 1 Gb modual they are about 13 quid last time I got a DDR2 . YOu will have 2 open slots and the other one should be able to take a a 1 Gb or 2Gb but you will have to check.
I have just done this upgrade for a mate and the change in performance improvement makes you look a whizz at tweaking the performance of a computer. I just stuck a 1GB and it makes the thing fly now that its not using the swap file just to get the OS loaded.