The way I understand it, dual channel mode offers more bandwidth between the processor and the RAM. Kinda like a RAID array within your ram. It is able to write to two modules in the same time that it would normally be able to write to one. At least thats just how I understand the way it works. In any case, Dual Channel will be able to outperform most, if not all, Single Channel setups with the same amount of memory.
Can't say exactly what they're thinking. Either might be right depending on how you look at it. But since you originally asked about two sticks of 256 or one stick of 512, definately go for one stick of 512. One stick of 1 gig would work better yet, but that wasn't your question. Another advantage of getting one stick of 512 is a better upgrade route to 1 gig, that is, you just get another stick of 512 and you have 1 gig. Also look for a faster CAS rate if you can afford it, like C2 instaed of C2.5.
I you are using your pc for just daily home use like surfing then it wouldn't really matter with one to get. If you're using it for gaming then go with 2x1Gb just for the heck of it.