Okay, so I guess I'll be going for a triple monitor setup, same question then?
Let me start by saying I'm no expert on multi-monitor setups, much less how well GPUs perform in these circumstances.
From what I can gather, you have several issues at play: high/ultra settings, Multi-Monitor (MM) setup, and single card vs SLI (for ease, I'll just use nVidia). Of course, remember there are some "fundamentals" regarding MM setups...like connectors (have to be DVI, HDMI or display port), and the fact that you need three identical monitors. Also, my research has indicated that GPUs with large amounts of RAM do the best with MMs.
With that in mind, I see three options:Second 660Ti for SLI
Cost ~ $300
This is the cheapest option. My guess is 660Ti in SLI should handle MM decently at medium settings. There's not much available showing GPU performance in triple monitor setup but I did find this article:http://www.tomshardware.com/reviews/crysis-3-performance-benchmark-gaming,3451-7.html
As the first graph show, at medium settings the 660Ti SLI performs really well - bump it up to High settings (4th graph) and the 660Ti SLI takes a significant hit. So if you want to do this option, you probably won't be able to game at ultra/high.770 gtx in SLI
Cost ~ $650 (385x2 less $120 from sale of 660Ti) - It's a guess but it gives you an idea
I selected this 770 because it has 4GB RAM which will come in handy for MMs. Unfortunately a direct comparison can't be performed, but I did find this article:http://www.guru3d.com/articles_pages/geforce_gtx_770_sli_review,23.html
I'm guessing they used the standard 2GB 770 so the 4GB version should perform better but don't expect miracles. I'd estimate another 5-10% bump in fps. Using the 770 SLI, I would reasonably expect you would be able to run MM with high settings at decent fps. Ultra...? You can probably enable some Ultra settings, but it's anybody's guess if you would be able to run every setting at Ultra. Plus, it would depend on the game.Single card 780Ti gtx
Cost ~ $625 (745 less $120 from sale of 660Ti)
This is slightly cheaper than the 770 SLI plus you don't have to worry about SLI issues. MPC provided some benchmarks in this article:http://www.maximumpc.com/nvidia_geforce_gtx_780_ti_benchmarks?page=0,1
Please note that instead of a MM setup, MPC used a 4K monitor. A single 4K monitor is actually more strenuous on a GPU since there are approximate 25% more pixels in a 4K monitor (3840x2160 ~ 8.2 million pixels) than in a triple monitor setups (5760x1080 ~ 6.2 million pixels). Here's another article which might provide more insight:http://www.anandtech.com/show/7492/the-geforce-gtx-780-ti-review/8
I'd guess the 780Ti and the 770 SLI would exchange victories and end up as a tie.
Hope this helps.