I believe most mining rigs (insofar as setups dedicated to mining alone go) actually have the cards suspended over the motherboard in a rack of some type, with the GPUs attached using a PCIe x16 to PCIe x1 ribbon cable.
As I understand it, it is not a high bandwidth application since while the GPU will be utilized heavily, the actual data throughput is low as the calculations take a significant time as compared to other applications like gaming that require processing a lot of data in real-time in order to render quickly.
Others welcome to correct me if I'm wrong about any of this.
The cards are plugged in with a x1 to x16 riser board using either a ribbon cable (old school) or USB 3.0 cable. I have a couple mining rigs for alt coins. You should not try and mine Bitcoin with a GPU rig unless you want to just throw a couple grand away for fun. GPU mining is for altcoins like Ethereum, Siacoin, Monero, etc.
This fad needs to end or vendors need to increase GPU production capacity. The upside will be the glut of used GPUs when miners get tired of losing money on their rigs and devices start turning up for re-sell on the used market.