If you already have 2 of the same monitors it' recommended to buy a third exact same model.
Your processor is on the lower end, but will probably be fine for another 2-4 years, you could try overclocking it a little.
One thin i'd definitly have bought ages ago, a SSD 128gb cost around 60-80$ nowadays, and thats enough for the Operating System and one or two games.
I'm not sure if SLI 520 works, AFAIK Nvidia Cards need an connector cable for SLI called the "SLI bridge" i think only high end gaming cards have these connectors.
Good news on this part, new nVidia Cards don't need SLI to run Multiple Monitors, a GTX960 can run 4 Monitors from one GPU.
And a GTX960 is the minimum i would recommend for gaming in 5760x1080 Resolution.
If you don't like nVidia anymore or would like to try out something new the AMD R9-380 would be a nice counterpart.
nVidia Surround or AMD Eyefinity work nearly equivalent fine.
greetings
Haldi
btw