Your GPU setup is quite powerful now, and I'm sure those reading this will be wishing that your Tri-SLI GeForce GTX 470 setup was theirs. For you, an upgrade is definitely a good path to get those frame rates up on your triple-monitor setup.
I'd suggest getting three GeForce GTX 670s, as you'd get a great price/performance ratio upgrade from them. You should see a great increase per GPU, and as a whole, a huge upgrade. I would say you should expect 50% or more in performance increases, as well as decreases in power consumption, heat output and noise.
The GTX 670s are great, and three of them would be quite the setup.