If you motherboard support Dual Channel, it's better for you two buy 2 stick of 512MB (best is dual channel or at least the two memory is identical). This is because you can run 128 Bit (128 channel for example when the RAM and CPU is doing the data transfer) instead of having 1 stick of 1GB, which only run at 64bit (64 Channel). If your motherboard has 4 DIMMs slot it's idea to go for dual channel if only two DIMMs slot, as others mention getting 1 stick of 1GB is also not a bad idea.
I personally like the OCZ, Kingston and Apacer. They all work great for me. Keep in mind both OCZ and Kingston has the manufacture lifetime warranty.
Hope this can help you.
Take care