I've never used Input Director or Multiplicity.

In general, when boxing a lot of copies of a game, the limiting factor is the amount of video ram.
So having a lot of video ram gets you the option for more eye candy enabled, and/or a greater number of clients compared to a card with less ram.

The benchmarks I've seen for the 670 placed it around 80-90% of the performance of the 680 (links were from Tom's Hardware).
When I got my 670, it was basically 66% of the price of a 680.
The Tom's Hardware video hierarchy recommendation of best bang for the buck was a strong factor.

My EVGA GTX 670 (4GB, Superclocked) could barely handle five clients on Ultra with DX 11, in and around Orgrimmar.
The game was playable on those settings, but in any kind of a challenging environment (raids and/or mass pvp) it wouldn't have been enjoyable.
The card itself wasn't pushed that far, in terms of graphical power, but it did use up almost all of the 4GB of its ram.
With DX9, the card didn't have any issues; I had reduced the settings to approximately medium/high and medium/low, as I was planning on 10-boxing on the one machine... never got there, with the Follow removal from battlegrounds.