I would like to add 2 x 512MB of the same size in the other dual channel. However Hynix memory is not easy to find. There is a sale on the same size of Ultra memory, but it is CL 3 with timings of 3-4-4-8 and 2.6v.
Stay away from the Ultra -- there's no reason to get memory that needs 2.6V instead of the standard 2.5V. Remember too that you should not add the new memory to the other channel, you want to put 1 stick of old RAM and 1 stick of new RAM in *each* channel, so both channels have identical configurations.