You folks are mixing and matching technology.

Any machine that supports Dual Channel DDR should have dimms allocated in matched pairs. If it doesn't support dual channel DDR, then you can (and often should) just buy it with 1 dimm, depending on how much they charge for the upgrade to 1 dimm. (Sometimes it's not worth it, then you go with the 2 dimms and throw one or both away when it comes time to upgrade.)

-David