SketchUp (
http://sketchup.google.com/) now owned by Google, and have a free version seems to be a popular CAD/3D tool at the John Salyers studio construction forum. However, be prepared to spend some time getting used to 3D modelling (although it is very easy compared to most 3D animation software), and measuring things out obsessively.
As for 'sketching' a rack, I'm not sure what you want to do. I don't think there is a library of popular rack unit face-plates, but again you can sorta plan it by measuring everything out, and marking things. FYI, a starting point is to have a box with 19" width internally, and figure 1.75" per 1 rack unit (1U).
However, at the end of the day, if you're just trying to sketch ideas instead of making a useful 'laymans blueprint', I think paper, pencil and some imagination will do wonders. Good luck! thumbsup