I've built several versions of roundhouses over the years and as many versions of turntables to go with it.  Here is a pdf document which defines a standard for a really big roundhouse done in a modular fashion.  Perhaps others will find inspiration in it.  

The Roundhouse Project
(~200 KB pdf file)

Every roundhouse needs a turntable.  Below are a couple of picutres of a turntable I worked on a while back:

    (click image for larger view)
Turntable Pictured During a build.  The track is cut at the edge of the turntable.  The turntable pit is about five bricks deep.
Turntable - Closeup A close up of the motor that drives the turntable.  The polarity switch reverses the polarity of power on the turntable track with every 180 degrees of rotation automatically.  

In more recent versions, I've used a circle of 9v track with a dummy 9v train motor to provide power to the turntable track.  This keeps the power cord from getting too twisted.  It also provided a more reliable support for the ends of the track.