I'm going through this as well. I'm happy with the performance I'm getting, I see constant 59/60 fps on my 3 screen Asus 24" LED monitors, but using TrackIR5 with the Track clip as a first timer has really affected my shooting accuracy as well as my situational awareness. I'm sure I'll get used to it, and it is said by all who use it to be superior to anything else, it just doesn't seem like it to me.
I had a "single card" solution running initially, a 690 GTX, but now have 2 680 GTX 4gb in SLI, so a total of 8 gb memory, which DOES make a difference in 3 screen, it does use the extra memory especially in some of the new games like Far Cry 3 and Skyrim, and even BF3 which is a year old now. I think the 2 680's are outperforming the 690 I had, but only by a slim margin.
The fact remains that if you try and do 3 screen on the cheap, with SLI/Xfire cards that are cheap, as in 150$ sort of cards, you probably won't be real happy with the results, unless you turn a LOT of the detail levels down in most games. A really fast single card like a high end 7970, 680, or 690, or better still SLI/Xfire with any of these other than the 690, is the best option if you want full detail and max FPS. I'm seeing 60fps maxed out nearly all of the time with all the new games, and with a game like Counterstrike GO, I'm seeing 300+ with the vsync off at times, and that's at ultra detail settings. The new Shogun Total war will really test this setup when you get a lot of troops on a field with a lot of stuff happening at max detail levels, but it's far from "chugging" still.