I have been thinking a couple of GTX 480's should do the trick.
I would suggest finding just about any other GPU setup than that, honestly.
The 480s are two generations old, and make better heaters than renderers.
The current "best bang for the buck" GPU list is here: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
If you want a lot of horsepower, the 670 is amazing, but if it's really just for a bit of modeling and running multiple monitors, a 7750/7770 would probably make life easier
I love the setup, by the way.