At a glance, the WZ has 1080p HD video support and 3 USB ports on the monitor, neither of which the other 2 have. Unless you're hooking it up to an HDTV or really have a need to connect USB devices through the monitor, it doesn't seem too useful. The other two look mostly comparable.
They all have DVI ports, so they'll all work fine with any of the Mac Pro graphics cards. For anything aside from 3D gaming, and high end graphics and video rendering (Aperture, Motion), you won't gain much from getting a fancier video card.
It will take more than watching DVDs and TV to take advantage of those 8 cores though!