Define better. 2x512MB should be faster as you can enable dual channel. (as long as the timings are the same, it should be faster then 1x1GB) 1x1GB however has better upgrading options. To get to 2GBs, you only need to buy one more stick of ram, and you'll only be using 2 of your slots. If you already bought 2x512MB, you'll either need to pull your ram and buy 2x1GB, or put 2x512MB in. This can cause trouble for some motherboards, as you need to slow the timings down if you run four sticks of ram.

So again, how do you define better?