First of all, an 850W psu should be fine. According to this post
http://www.tomshardware.com/forum/335793-28-rail-question the 670 uses 15A@12V; your PSU supplies 70A@12V, even if you fudge the math up 20% you're looking ok (3x 18A + 15A for CPU/misc is 69A). If you plan on doing any overclocking, you may want to go a little bigger though.
Have you done your homework on 3d-multimonitor setups?
I have not, but I would make sure that gaming in 3d across 3 monitors would work as you hope, as you are making a huge investment in something a very small number of people have tried.
As bigbase suggested, it maybe worth getting a PCIE3.0 mobo (which means ivybridge CPU as well), as 3x670s are fairly likely to actually take advantage of the extra bandwidth provided.
I'm not really sure that the i7 is necessary for gaming, even in your extreme case. The only tax on the processor is trying to hit 120fps, hyperthreading won't really help you get there, so an i5 will likely give you the same performance; the rest of the burden is on the GPUs. (of course if you are doing more than gaming, then you may have need of the i7).
edit: Also, that is a pretty awesome setup; I wish I could get approval for disposing that much income