No idea how rendering is like in Sketch up but for a general low budget workstation:
Core i7 or AMD X6
At least 4GB DDR3 RAM
Samsung F3 500GB or 1TB, depending on need
A good QUALITY PSU like a Corsair 650TX or XFX 650W.
A decent case like an Antec 300/900,etc
DVD RW, card reader, etc.
As for the graphics card, that depends on if this is a true workstation build or a gaming + workstation build. If this a true workstation get a workstation graphics card, if not get a good gaming card like a GTX460.
Again, I don't use SketchUp, but I do use Maya,Inventor, SolidWorks,etc and for these software, for small assemblies/renderings you really don't need a workstation graphics card.