One thing I learned doing this is to never set a deadline, it just never works :P
If you want to do a true surround sound setup, you're going to have to rewire the entire stock speaker system and how you would still use the factory amps I'm not too sure.
The highs are prewired for left front, right front, left back, right back but the lows are going to be a pain in the arse. From what I remember they are wired as just a left channel (powered by one amp) and a right channel (powered by the other). With the factory wiring, you cannot separate front from back channels I believe.
What you could do is keep the factory wiring as is. The lows would remain as is (left and right channel) but the highs could be true surround. Get a 4 channel amp and wire it up. The audigy NX has outputs for each channel including subwoofer and centre (these are on one output on the sound card, the left channel is subwoofer and the right channel is centre since they are both mono, no need for 2 outputs on the card).
If not, have fun trying to figure out the wiring