Creating a GPU cluster

How can you add more PCI slots to a motherboard? I've seen people use cables to put the cards outside the box. But the closest I've seen is PCI riser cards but that only converts 1 slot into 2. So how are people getting 10-20 cards in a single box?Edit: I know GPU mining isn't profitable anymore. Doing some parallel programming with CUDA and remember seeing lots of threads here last year about mining setups. So figured people here would know :) via /r/CryptoCurrency http://bit.ly/1Qb3OFs