I would like to clear something up for you that you may not know, if you do then great.
There is no such thing as Special "Dual Channel DDR Modules", there are Matched pairs, and these modules are garunteed by the manufacture to work well together.
As of now these companies are charging extra for matched modules in most cases, some are not however.
All you need to do is buy two modules that are the same, just buy 2x512MB or 2x256 of whatever brand you are wanting to go with.
I Hope I have cleared up any misconceptions you may have had, if you already know about this then good deal.