That's correct, nForce boards can fully function with just one stick of RAM. However, to take full advantage of nForce dual channel memory interface, you would need to insert two sticks of DDR DRAM for a 128-bit bus to be possible. This is really only necessary if you're going to utilize the onboard GeForce2 MX video.
Otherwise, as long as you have the most recent BIOS, it shouldn't matter how many sticks of RAM you have.