In general - Dual Channel is a good thing -- it allows the machine to address two memory modules together as if they were one large one. The theoretical improvement in RAM access performance is around 30% -- BUT the real-life improvement in software performance is much less. Maybe 5% overall, but that's just a guess because it is highly dependant on what programs you are running and how they use the machine.
Now, your present order:
It depends on how much money you have to spend and what you want to do with the machines.
If you are buying the RAM from Apple, cancel the order ASAP and reorder them with the stock 512.
Then for the same money or about $20 more, order 1 Gb SODIMMs from a reputable third party supplier. This will get you 1.5 Gb of RAM total, which will give you greater performance than having 1 Gb in Dual Channel mode. It will make a difference to applications running under Rosetta especially.
You then have the option of going to 2 Gb (matched 1 Gb modules) later, and you only have to sell off one 512 Mb module, not two.
Starting off with both sockets filled with 512s is a dead end.
But this is still dependant on your budget, and on the programs you will be using. I use Web and Graphic and Audio applications, so I wouldn't even try to run a machine with less than 1.5 Gb RAM. If these are only for word processing and email, 1 Gb may be OK.
In that event I still lean toward going with a single 1 Gb, leaving your options open. Of course, as above if you are buying Apple's 1 Gb, then you could be getting 1.5 Gb for about the same money third party.
