welcome to the community bud.
That machine will be great for it, dont expect perfect frame rate at max graphics or nothing, but by far will make your gaming enjoyable.
The two side monitors in portrait mode seems to be a pretty popular way to go at the moment, and im sure if i was redoing my system i would look at also doing this.
go for gold man, go 5 the games much more enjoyable when your doing everything yourself and is a true accomplishment when you can solo all the 5man content.
As for the second video card you will require this for a 3rd monitor, im not farmiliar too much with the triplehead2go card so i cant really help you there. The extra memory would be fantastic, although i used to run 5 instances with only 2g and it was ..well..ok to play
Gl with it all keep us posted.