In order to use Dual Channel Memory, your motherboard has to be capable of supporting this technique and you will also need two equal memory modules.
Best to check with the motherboard manufacturer.
You fit it in pairs normally the first pair would be fitted in slots 1 & 3, second in slots 2 & 4, most manufacturers actually colour the slots so you can't mix them up.
You would have to check with your motherboard manufacturer to see what you can and can't do. It is possible that the 2nd pair you put in could be 1GB each, it's the pair that needs to be matched, not all 4.
My advice would be if you feel the need to get 2GB's of RAM don't mess with 4x 512mb sticks just get 2x 1GB to start with, as filling all 4 slots with relatively small sticks means that should you feel the need to get more RAM then you will be removing perfectly good sticks which is a waste of money.