Weighted dips at home I do on parallel bars because I have the setup for that. If I had the space for weighted rings dips I would do that instead.
My maxes are:
weighted dips: +170 lbs (~6-8 weeks ago)
weighted rings dips: +150-155 lbs (~16 weeks ago IIRC)
Once you try rings dips you'll see that as you get the poundages up that they become really stable (except in the bottom portion of the dip).
Bar dips (horizontal bar) are much like bulgarian dips which Coach Sommer put out an article on. Go read it on his site... http://www.gymnasticbodies.com/routine/archives.html