The best way i found to get around this are to use outboard sound processing. I have an nforce board as well, and it outputs 5.1 coax to my logitech Z system. the logitech reciever then decodes it into 5.1. It works really well for turning stereo sources into 5.1.
When running in 4 speaker mode, your basically duplicating the front channels onto the rears, so it's not true surround sound(but it is quadraphonic). I recall using apps in the past that could force the hardware to output this instead of basic 2-channel stereo. I have an Asus A7N8X-e, so we have almost the same mobo(5 years old!). You actually have all the same hardware i did about 2 years ago.
how are your speakers plugged into your computer? Are you using basic 3.5mm jacks, or are you using the coax outputs? Usually, the 3.5mm jacks are dependant on whatever the source material is comprised of, espically in the case of games. I know there are apps out there that can "expand" it into more channels, but i don't remember the names.
Lee