Your best buying a pair of 512mbs or a pair of 1gbs mate, then you can run it in dual channel mode, which gives a performance advantage.

If you look at the suggested memory at crucial it will say 1gb kit (2x 512mb) or a 2gb kit (2x1gb) which is matched pair memory suitable for dual channeling. The 2x256mb sticks that are already in there will be a matched pair set to run in dual channel.

Depending on the board, slots 1&2 and 3&4 will be the linked slots for pairing, or on some machines such as HP/Compaqs, it will be 1&3 and 2&4

hope this helps