Yes, nForce2 supports dual channel. Yes, it will work with an odd number of sticks - why have an odd number of slots if it only works in pairs?
However, to get dual channel, you need at least two sticks of RAM - 2 OR 3 sticks will get you dual channel, not just 1. Just so that's really clear.

Also, beware of so called "dual channel memory" - it's a marketing gimmick. A single stick of memory cannot be dual channel, because it is the design of the memory controller on the motherboard (or in new systems, onboard the CPU) that determines how many channels are available.
Yes, there is an improvement with dual channel, though with the nForce2 without the Integrated Graphics Processor, it is probably less than a 10% improvement in memory bandwidth. With the IGP, it offers considerable improvements, as the IGP has its own channel through which it can access the system memory. But the latter is probably not a concern for you right now.
Abit's NF7-S rev2.0 was a pretty darn nice board; I have two systems, one with a rev1.2 and the other a 2.0, and both are quite good performers. The NF7-S rev2 is not to be confused with the NF7-S2, a different board, and not as tweaker friendly.