[s-cars] Alternator Issues

[everett2487 at adelphia.net]

Hey guys,
Recently I made the long drive from Maine back to school in Virginia and the s-machine was great.  However a few days after I got back I went on a beer run, only the second time I drove it since getting here, and couldn't even make it to the store before the car was about to shut off, lights severly dimmed, car was sputtering.  So I am thinking its the alternator.  
I read the recent alternator woes posts and am hoping that my problem will be similar to Matt's, since I dont want to be spending all my beer money on fixing my alternator.  I removed the brush assembly and found that one of the brushes was past the wear mark:
http://img178.imageshack.us/img178/1785/dscn0551adz2.jpg
http://img178.imageshack.us/img178/6796/dscn0550ake3.jpg
Could this cause the car to go from running fine to almost dieing in a 10 minute trip or do I have another problem?  Is there a way to test if the problem is the brushes, or the regulator, or just the alternator itself?  
Thanks a lot,
Chuck
'93 S4