Upon someone's recommendation, I used Google Sketchup:
http://www.google.com/sketchup/downloads/It's great to get the overall feel. I had NO idea how to use a CAD program but the tutorials really help and it saved me days of trial and error. I will caution, though, that you'll find yourself tweaking too much in this when, after a while, you just have to start cutting the wood.
For my CP and Marquee art, I just used Photoshop, 300dpi. I saved it as a flattened Photoshop file which is a fraction of the size of a Targa or other high end, no compression file... plus, it's the original format and Mamemarquees and Emdkay read them both. I used both guys for my CP and Marquee, respectively, and I really dug their work.