That question is very motherboard dependant. However, most MB's I've seen are capable of running single channel with 3 strips. Some memory configurations force 333mhz though, which in your case doesn't seem to be a problem at this point.
When/if you upgrade to 400mhz memory though you'll have to pay attention to single sided/dual sided module configuration.
Just so you know, there is no such thing as dual channel memory. The single or dual channel comes from the memory controller.
Also, if you find yourself wanting or having to purchase more memory, then I would probably recommend going to an AM2 platform instead of sticking with s939.
I am in the same situation myself and will be building a s939 machine in the next month or so, but I am doing it because I have just about everything I need to build a new machine laying around including pc3200 memory. If I were going to need to purchase memory, then I would go with the newer platform and DDR2.