My development/rig hardware:
Additional info on the GTX980 platform is available here.
The Asus motherboard supports four 3.0 x16 PCIe slots, as well as two 1GBit ethernet ports. Having a dedicated ethernet port for the radio is almost mandatory as the raw DFC samples pretty much saturate a NIC.
The AMD EyeFinity supports up to 6 monitors, although I have only used 3 DisplayPorts at once. Get the ACTIVE adapters! The onboard Intel graphics supports 1 DisplayPort, 1 HDMI, 1 DVI. If that will meet you needs, skip the EyeFinity. I needed 3 DisplayPorts, and found that adding a 2 port AMD card caused no end of issues. Ubuntu doesn't seem to support concurrent use of the Intel and AMD drivers properly. Using the AMD EyeFinity alone (nothing attached to onboard/Intel ports) works flawlessly for me!
Don't skimp on the power supply, A CUDA card consumes a LOT of power.