Id like to put another vote for Google Sketchup. It takes a bit to get used to but its much easier than some of the more high level 3d programs.
I used microsoft viso to make a side view. This helps get angles and such. Then transferred everything into Sketchup. I personally think it is easier to put things in sketchup if you have an idea of the dimensions first.
One thing I would suggest if you have the time is to do a mock up. When I made my first cab, I went to home depot and got some insulation panels. They are 4'x8' panels the same size you would use with wood. But this are made of foam with a coating on the outsides. Super light and only like 2 bucks a sheet. They cut very easy with a razor blade and you can attach pieces together with normal nails and just push them in. This is a good way to see if the height is right and if the angles are good. This helped me realize that my control panel was too short for me and the tv angle would be off. Keep in mind that this is just a mockup and cant hold any weight but its better than nothing and more realistic than using cardboard.
Hope this helps.
Chris