I want to use a PC as an audio source (with an average quality
soundblaster card) for a stereo system.

My amp is about 100' away and attaches to the speakers (for various
reasons, can't move this).

Two questions:
1. How much will the sound quality be affected over a 100' cable?
(i.e., is this an acceptable distance?)
2. What type of cable should I use -- any special considerations or
will anything do?

Thanks!

You will get less signal loss with larger conductors. You will pick up
less interference with twisted or shielded cable. I would go trial and
error with a piece of speaker or networking cable and see how it
works.

Monster brand cable is great stuff. Visit their website at
MonsterCable.com and look around.

Optical cable for transmission Audio-signals : 

Premium Gold TosLink / SPDIF Kabel 15m
http://www.lindy.com/de/productfolder/03/35/35246/index.php
25 Jahre Garantie (Garanty) : Verfügbar in Längen (buy-Length from)
von 0,5m bis 50m.

http://www.hifi-regler.de/shop/oehlbach/oehlbach_coaxial_optical_converter.php
http://www.al-kabelshop.de/katalog/WebTips/Index0.html





I recommend to send over far distances the music signal "optically".
Then it is also not affected by voltage differences, Interverenzen,
electrical fields, etc. Ever at the cable end and at the cable
beginning, it needs however an adapter, the light signals again into
current signals converts and in reverse. For this adapter also a mains
connection is mostly required.
If you send however audio signals of the PC on the Stereoamp, and the
voltage and the amperage are highly enough (>12 VDC), then need you
possibly no optical cable. The signal transmission becomes the more
susceptible, when the signal is the weaker. Surely an attempt with a
normal 50 m cables cannot harm . Greeting.


://www.google.com/language_tools?hl=de (Translation per Google)


Maximal cable length for different systems (in german) : 
http://www.winpage.info/pages/Hardware/Kabel/kabel_laenge.php
Example : 
magn. Plattenspieler/Mikrofon ohne Verstärker = 3m 
DIN-Ausgänge, Mikrofon mit Verstärker = 10m 
Cinch-Ausgänge = 20m 
Lautsprecher (Speakers) = 10m 
Symmetrische Geräte = 1000m

Thanks for the help everyone!