I urge you to read through all the posts in this thread
http://www.chiefdelphi.com/media/photos/35696
That was the first sheetmetal chassis I tried to make, so the information people posted in there is very valuable, I know it helped me a ton. Since then, I've been lucky enough to learn from people like James Tonthat, Sean Cantrell, all of JVN's CAD as well as from my current employer about sheetmetal design.