if you have a 1000W system, what GPUs will give you > 2 MH/W?
Well I don't have an exhaustive list but 5970, 5870s, and 7970s definitely will. A lot of other cards in the right config likely will too. The other half of the equation is things like using usb stick instead of HDD, turning off the "junk" you don't need in BIOS, single core Sempron, using linux over windows, 80-Plus Gold PSU, etc.
So it is more like:
a) choose cards with high internal (GPU only) MH/W
b) get a large amount of MH per rig (to ammortize the system load over large number of MH). >2GH should be the goal.
c) build the rest of system smartly to minimize non-GPU load. The largest being low watt CPU and high efficiency PSU.
Over time (like any other commodity business) it will require more and more capital/work/thought/design to stay ahead of the efficiency curve.