50 feet is not too long at all but there are some issues with Svideo you need to be aware of. Mini coax is rather lossy. And some really cheap cables don't even use 75ohm coax.
BEST:
Use standard RG59 type such as Belden 1505 (1694 is great but a bit overkill IMPO) BNC to BNC cables and put Svideo to BNC breakout adaptors on each end.
BETTER:
Use true 75ohm mini Svideo cable.
Here is the perferred AVS member source (and I highly reccomend them too)
http://www.bluejeanscable.com/store/svideo/index.htm
Above all stay away from Monster or other brands that don't specify the type of raw wire or the technical specifications of the wire they use.
Now the nest question: What is the PC doing with the video? If just a TV window to watch while you are working, then a lower cost cable may be OK for you. If you are capturing and editing the video, then the highest quality should be used.