Yeah get the second FTW. The sad part is with these current settings; most of the time i dont really break 100fps or any triple digit framerate. Usually i have to turn down MSAA or play at High Settings to get the 120fps @ 120hz power from these two 670 FTW's. The 780 is about 75% the power (this is an estimate) of two 670 FTW's.
I have seen some voltage modded 780's at 1.31v that come very close to my 18,000 3d Mark 11 Graphics score , around 16,000 or 17,000 but they probably have liquid cooling full blocks on their VRM's and GPU Core. So yeah, you can get a 780 and voltage mod it to come very close to the power of two 670 FTW's but you really have to push it.
I dont think you can break the 100fps average with a "single 780" at 4x MSAA Ultra Settings, 100% Resolution Scale" so if youre planning on doing that go with the second 670 FTW.