you'd get better performance if you had the RAM working in dual channel
get 2x512 sticks if you really crave performance, or just get whichever is less expensive (2x256 or 1x512) if you're worried more about price
although sometimes you get errors if you mix 2 brands of RAM
OK, if I understood your question, you get the same "performance" from 2x256 as you do 2x512. RAM stands for Random Access Memory meaning it can access any data located on the stick at the same speed as any other data located anywhere else on the same stick. If your looking for "performance" you should check the "CAS latency" of the ram you intend on purchasing and make your decision based on the use of your computer. If your into playing graphic intensive games like Far Cry I would recommend purchasing something with a "low" CAS latency and would go with 2x512 and set them up to use Dual Channel if your motherboard supports it (to answer another question, use the same brand, model number, therefor the same latency when using Dual Channel). If your motherboard doesn't support dual channel install like you normally would.
If money is not an option I would recommend buying the following (remember to check your motherboard QVL for information on supported RAM before purchasing):
If your motherboard does not support dual channel go with one stick of 512MBs, takes up fewer slots and it would just the same as 2x256MB modules. The system sees the memory as a whole and not as individual modules. If the system runs in dual channel mode then 2 more 256MB modules would be good or 2x512s. Sure lower CAS latency will help with speed but not much as people are led to believe. The difference between CL-2.5 and 3 is about 8 nanoseconds, can you see the difference?? If you can see a difference of 8ns, I will call you superman.