I've read in several places that you shouldn't use S-video cable runs of more than 35', although I believe that some people on this forum have gone up to 50' without any problem. YMMV. Your 20' run should be no problem whatsoever as long as you use halfway decent cables.
As someone stated earlier in the thread, you don't have to pay an arm and a leg for decent cables. I'm using a 12' S-video cable to 4-way switchbox, then going to a 15' S-video cable to the X1, and it looks great. The total cost for the 2 cables and switchbox was less than $40.