that depends on whether your motherboard supports dual channel memory. If it does, then you get a huge performance boost by running 2 matched 512MB sticks verses a single 1GB stick.
With dual channel, the system uses both memory sticks at the same time, doubling the memory bandwidth, this is clearly a much more efficient setup
With single channel, the system doesn't use the second stick until the first one is fully utilized, so with a single channel setup, it really doesn't which setup you choose
hope this answered your question