While it's counterintuitive on the surface of it, nForce2 can indeed run three modules in dual-channel mode, or dual-DDR mode to be more precise. I talked at more length about it in this
thread.
Bigger picture: on nForce2, for nearly anything a home/enthusiast user is doing, dual-DDR is not going to boost performance by more than 2-5%, except for the special case where the person is using nForce2's onboard video, at which point you can expect large improvements in 3D acceleration for gaming compared to a single DDR channel. I use just one memory module on my A7N8X Deluxe at the moment, and if I add more, it'll be the increased
amount of memory that I expect to yield improvements, not the dual-DDR aspect of it.
So don't worry about it

Plug in your modules and use 'em.