I doubt you'll need a line driver with the Audigy 2ZS.. I have that going to my amps w/out a line driver, although some people live and die by line drivers, so who knows.. It depends how high your noise floor is to begin with.
Get that down first.. Make sure your grounds are GOOD.. REALLY GOOD! That way, no power will run into your computer/speaker setup, and you won't get any "white noise" or background noise aka "line noise".
Also, if your computer is in the trunk with your amps, then you will be lucky, like me. You will have really short RCA runs (mine are 2.5') so you won't get much interference. You can buy double-shielded high-resolution video RCA cable, too, and put some good ends on it. It will take a while, but that RCA cable is good stuff.
Or you could do optical out, but I doubt that would help much.
Get good components up front, and align them properly for best imaging. To get good imaging, talk to the guys on caraudioforum.com, and get the speakers in phase at your ears. You may need a time delay device (sound blaster has this in their software!!) or you may need to swap the phase on one side of speakers in your car.
Get a good crossover to kill some spikes and resonances, and get the signal really flat, for good sq..
I hope this helps. Let us know how it turns out!