rupov is still a baby, meaning, i only started work on it very recently and it's never had any releases. anything that is currently online ( http://www.darkarts.co.za/projects/rupov ) can be considered unstable and the api or even the design may be scrapped at any minute if i think it can be done better another way.
Povray is very easy, I would also recommend you have a look at it. If you use a unix operating system (or a unix clone like linux etc) then look at "kpovmodeller" it's great ;)
rupov will be designed as an interface to povray, which is a raytracing engine. rupov will be usefull for creating rendered scenes that are non-interactive. For example I am using it to generate povray scenes for tiles in an isometric game I am making. rupov generates the graphics at 'compile time' which are then just drawn ingame.
Clive Crous on opengl
If you want to learn graphic programming, in the sense that you would like to create interactive scenes that respond to user interaction, then opengl is the way to go, and for ruby I reccommend 'rubygame': http://rubygame.seul.org/
It is also a new project, the latest online version "1.0" does not include OpenGL, but the OpenGL code is already completed by the author and is available through his repository. He has said on his mailing list that it will be released any day now, for which I am waiting with much anticipation :D