Adding ram

I have the following:

g4 ibook

cpu speed 933 mhz

L2 cache 256 kb

memory 640mb, 128 built in and 512 ram

bus speed 133 mhz

I understand that my ibook can have a total of 1gb of ram. Do I buy 512mb card and add that or do I buy a 1gb card and replace the 512mb?

Thank you!