The 4x512 will run faster, since the sticks are smaller and you'll probably be able to run at least 2 of those in dual-channel. It's like having to search through four thin volumes, rather than 2 phonebooks.
However, you can reverse this by choosing a dual-channel pair of 1gb sticks that have ultra low latencies, so the extra speed will actually make them perform better. The caveat here is that the faster the RAM latencies, the more expensive they are. You could easily spend over $200-300 on a matched pair of low-latency 1gb RAM sticks.