I learned SketchUp last semester, and have been using it to mock things up that I'm making. I initially was going to post an instructable on building a desk, but there's already several published which are very good.
I sketched out my desk on graph paper first (to scale). Then I built the desk (minus doors and monitor stand). I spent an entire semester with a low monitored desk. After the semester I sketched my desk in SketchUp and started modeling things on the desk so I could figure out how to organize things. Finally I figured out my eye height, and modeled the monitor stand, then built it.
The desk is made entirely out of 1/2" plywood and 1 1/2" nails. The monitor shelf is the same plywood on two by fours.
After I placed my monitors on the shelf, I realized I needed one other thing. So I took the shelf back into the garage, and sawed an eighth of an inch off of one of the legs, so that my cutting mat would fit underneath. Overall, it works perfect!
The desk holds together quite well, and I move it around a bunch when I need to deal with the wire mess behind it. Not only does it hold together quite well (I've lifted it on several occasions), but since I built it to fit the specific things on each shelf, everything has a very nice placement.
If you'd like the SketchUp file, message me and I'll be glad to send it.
If anything is difficult to see, you should be able to click the image for a larger version.