Hi! I had the same problem with an ATI HD 4500. Dont know either how to force 640x480 or 640x400 resolution (besides that "dune2000.exe -f" should use 640x400). I realized that I didn't have 400 as an resolution option at the catalyst control center, so I added it as custom resolution with Powerstrip (just goolge it) and since than no black bars no more. Strangely with compatibility settings Win95/98 they still appear, with Win2000 or higher or without compatibility settings it works fine. Maybe this will do the trick for you too. Good luck!
