Does your stereo have a digital coaxial input? Does your PC have a digital output? If so, that would be the best way to go.
However, my personal experience with a 50 foot run was quite good. I used to have my PC connected to my stereo over 50 feet of cable and it sounded great. I have since purchased a receiver that streams music over my network and it does sound better than the 50 foot analog run.
If you have no option other than the 50 foot analog cable I would use RG-6 coax with RCA terminations. You can make this yourself or you can buy a pre-made one like
THIS.