I'd take the money I'd waste on a Mac, and spend it instead on a computer with a least three free slots [either PCI or PCIe], three identical video cards [nVidia, ATi, or Matrox], and three rotatable [swiveling] VA-panel [MVA or PVA] flat screens, which can [physically] rotate [swivel] between portrait and landscape mode.
I wouldn't waste any other money on the "chassis" or any time or effort worrying about the operating system.
Once you've experienced "large" real estate on the desktop [and the enhanced productivity which results], you can never go back to that claustrophic feeling of one tiny little monitor.
For more on researching LCD panels, I'd visit these sites at a minimum [the key words you need to learn about are "panel lottery", "ghosting", and "viewing angles"]:
Anandtech Forums, The LCD Thread
A List of LCD-Units in Monitors
DigitalVersus.com, Product Face-Offs
Finally, before I purchased my monitors, I'd find it at NewEgg, click on "read all XYZ Reviews", choose "100 per page", and "Sort by lowest rating", so that I could learn all of the worst things about the monitor before I purchased it.
That's nice... the Mac Pro supports eight 30 inch monitors... and has the wattage to power the graphic cards necessary.