A friend of a friend of mine can bend sheet metal so my mining friend and I came up with a design. The case would be a standard sized 19" 4U rackmount case. It could fit 4 high ends cards comfortably or 6 cheaper, lower powered ones. The cards would be situated above the motherboard at a height just above the stock sempron heatsink. For cooling, the back should be able to fit 3 120mm fans with the power supply oriented vertically or 6-7 80mm fans depending on the PSU. of course, this fan takes care of all your problems: http://www.newegg.com/Product/Product.aspx?Item=N82E16835706015

We never ended up making it since the availability of good cheap cards ran out *cough5830cough*