I would say 2x512Mb sticks because you can always add two more later. You will be wasting money later if you are replacing. Unfortunately, there are always drawbacks. A chain is as weak as its weakest link. Same with memory, memory is as slow as its slowest chip. I think you are more likely to get sticks that will not overclock as well. That seems to be how it is with 512Mb sticks of RAM.