Wait a minute, someone's confused. An optical output is a type of digital output, and the main purpose of a soundcard, at least as far as music is concerned, is to act as a DAC. Also, the Audigy 2 ZS' digital output is optical. To utilize it, you need an optical cable with a mini optical plug that will fit into the headphone jack. And, the line out you are talking about (what you would use a mini to mini cable for) is an analog signal, so you would be bypassing the total bithead's DAC and just using the amp. It would do nothing more for your computer than the total airhead would in this configuration.
If you get an external DAC, no matter how you hook it up to your computer, your soundcard won't be doing anything desirable to the digital signal. In the case of the Audigy, you would be better off bypassing it completely (via the USB port) than using its digital output. This is because it doesn't output anywhere near bitperfect: It converts the signal to 48KHz, and does a very poor job of it, severely damaging the digital signal. With the right software, you can get a bitperfect signal from a USB port, which is what you should do if you get an external DAC. I actually bought an Audigy 2 ZS intending to use it to hook my new Lavry DA10 up to my laptop via the optical output, and now I'm having to buy something else with a bit perfect digital output since I found out about this.
I wouldn't bother with an external DAC, though, because your soundcard will work just as good as anything you would get in your pricerange. I don't know what amp to recommend to you: Perhaps a Total Airhead? Good luck!