The modules that you bought will work for dual-channel mode, no problem. Just leave the middle memory slot vacant. The board has two 64-bit controllers, one hooked up to one slot, and the other one hooked up to the remaining two slots. They don't actually weld the two memory busses together to make a single 128-bit bus, but rather they load-balance between the two 64-bit busses. This is why you can run dual-channel with
three modules on nForce2
About the FSB speeds: 166MHz FSB is double-pumped, so when people say they are running 166MHz or 333MHz FSB, they're saying the same thing in two different ways.
Regarding the onboard audio, why not try both ways and see which one you like better

If you do, I recommend using your Santa Cruz in the fourth slot PCI from the top and disabling the onboard MIDI, onboard gameport and onboard audio processing unit (APU) in the motherboard's BIOS, so they don't duplicate eachothers' efforts.