The TWINX512-2700LL are two CMX256-2700LL sticks that have been tested to work at a high level together. If they pass the tests, they are phisically packaged and sold together, specifically for dual channel motherboards. If you have a dual channel mobo, I'd get the TWINX ones. If not, I'd get the single CMX512-2700LL stick. Then again, if you plan on buying a new mobo in the near future, I'd probably get the TWINXs anyway. I don't think the performance will be that different in a single channel mobo, but I'm not too sure.