Check out the Crucial website. Has a little tool that will tell you exactly what how much ram you can use, in what config and is very reasonably priced:

http://www.crucial.com/uk/mac/index.aspx

If I remember right tho, you can have a max of 8GB using two mem slots. So it depends what you have in currently to what your next step is.

www.mrmemory.co.uk

Best memory people I've used - for both business and personal.

THanks guys - went with the crucial one as they had a little program that worked out exactly which memory I needed - and only £20, happy with that