Personal preference really. WoW and most of the multiboxing solutions can adjust to whatever resolution you want to run. Performance wise the only real impact is that more pixels = more work for your video card(s). If you're looking at one of the larger format monitors (the 2048x1152 may count), then be sure you look at what it takes to plug it in - i.e. 2560x1600 monitors often take 2x DVI ports to operate at max resultion.

Whatever looks best is really up to you. Personally I'd get two matching monitors for a cleaner look.