Set your windows driver to NOT hide display resolutions your monitor can't show...

Then check what resolutions WOW thinks are available... set it in windowed mode so it doesn't try and go off the charts... then pick one of the 16x9 (Wide) resolutions... I had 1280x1024 on my main screen and something absurd like 1600x900 for the top and bottom of the second screen, I've since reformatted due to a crashed raid and dont have the exact settings...

On my system in XP I had threeboxing setup pretty nicely... The little windows app you see is a quick and dirty launcher I wrote to cover the things I had to dig into most often.