So I gather the PC worked fine with 2 512mb modules installed and the 1gb module installed but when you installed the fourth 1gb module, the PC blue screened, correct?
This is what's happening to your PC.
With the just the 2 512mb modules installed, your motherboard is accessing them in dual channel mode. When you added the first 1gb module, the motherboard switched to accessing the RAM to normal mode (no dual channel).
Now, when you added the second 1gb module, the motherboard tried to switch back to dual channel mode, but because the RAM modules are not all the same size, the PC blue screened.
Remove the two original 512mb modules and replace them with the two 1gb modules. Your PC will access them in dual channel mode and all will be well.
Two gig of RAM is usually more than enough for most users but if you want more, you will have to either buy two identical 2gig modules, or four identical 1gig modules.
Sell the two 512gb modules.
Good luck!