I've recently found a free oscilloscope program, and would like to use it to find at what gain my amp will start to distort.

How should I go about setting this up? I would use the 'Line In' jack of the soundcard, but I know that it is not a good idea to put more than 1 or 2 volts in there, I also don't know the impedance offhand, nor where to find it.

I am potentially going to reach up to 50V @ 2ohm.

Is this even feasable?
Thanks