Hi,
So you have 6 Monitors! This makes things complicated. IF you want to run Eyefinity aka Single Large Surface aka maximize windows(games) over all 6 screens (in a 3x2Landscape I assume) you need to connect ALL 6 Monitors to one card!
Even(especially) if you run Crossfire.
To connect 6 Monitors you need a Special GPU with 6Connectors. There is a thread from suiken_mieux which lists a lot of them.
http://www.wsgf.org/forums/viewtopic.php?t=24334The OTHER way would be MST Display port Hub, I.e from Club3D, that has 1DP input 3DP output.
Depending on the card you will get you could use 1x DP for the Hub, and then DVI-DVI-HDMI for the other 3.
For the Performance.... 3x2L 1920x1080 per Screen? i'd recommend Crossfire R9-290(x) everything below wont bring enough performance for BF4 or similar games. Except you play on ultra Low settings.