If possible I would pull a 14/3 (12/3) from the existing switch to the the new location and do a dead end 3way.
Failing that, all you need, in an ideal situation, is a single pole x10 switch at the existing switch location and a wall controller with a 1 button keypad at the new location. The controller wires right to 120V and sends its signals out over that leg. If your switch is on the other leg you may get iffy response out of the switch without installing a phase coupler and maybe a signal amp as well. Depending on what the owner has plugged in (noisy motors and compressors, filtering power supplies, cheap electronic ballasts, etc...)you may have problems with the signal either being drowned out by noise or shunted to ground by a filter that that mistakes the signal for noise. And you always run the risk that the owner will purchase something that interferes with X10 after you install it. Oh, and if there is no means in place of isolating the owners X10 system from the utilities transformer you might be sending your signals to the neighbors and vice-versa.
It can be a great system but it can drive you crazy too.
I try to avoid the $12 WS467's as they don't have a neutral and must trickle a current through the load to power themselves up. I have also had random ON problems with them in the past. The only reliable switches I have found with a neutral are decora style although I remember a post here referring to good toggle style switches. The dimming switches are nice if they are reliable but I have found the 20A contactor type to be the best, but they turn on and off with a loud SNAP. Of course if your switch is backfed you won't have a neutral available. In that case you may be able to install a fixture module in the lighting outlet box but they often don't have enough room for that.
If you plan to do much X10 work I would buy a few things to keep on your truck that you can use to pre-test an installation. A plug in mini controller for about $10 and both screw in and plug in lamp modules can tell you pretty quick if its going to be a challenge to make it work. Just remember to have the owner turn on every light, tv, computer, printer, etc.. that might interfere with X10. I learned that by spending hours getting a complicated system running and getting a callback. Turned out a bank of fluorescents that was on often at night was killing the signal. It worked fine when they were off.
Last edited by ChicoC10; 10/30/07 05:39 PM.