dual GPU cards, 1600 stream processors or more per GPU, 64mb memory clocked between 200-300mhz, pci-e x1 (intead of x16), and reference type coolers.
dual GPU cards are the most efficient in terms of space and power because they share the components on the board, like memory, vrm's, and such. majority of hardware mining performance is based on stream processors, currently the highest is 1600sp from the 5870, so either match this amount or higher. the upcoming 7000 series cards will have a 28nm die shrink, so they should be able to fit more stream processors in the same amount of space. mining does not use a lot of video memory, so as little as possible while still being operation. i think 64mb is a good medium. lower memory clocks helps with temperature and power consumption. and lastly, with reference coolers, there's directional flow of the heat. it all gets exhausted to the rear (or out to the back of the case if it's enclosed), where as non reference type of coolers blow heat in all directions. when you stack these in a mult-GPU setup, they use each other's heat, and usually the top card always has the highest temps.
btw janathan, are you still mining with those 6950 setup you showed me? or are you done with that "experiment" and only hosting Bitcoinduit now?
Also, design the optimal graphics card so that:
* It pulls most of the current on 12V from the PCIe 6-pin and 8-pin connectors, instead of pulling it from the slot. Otherwise you have to perform this kind of hack to properly power an 8-GPU config: http://blog.zorinaq.com/?e=44
* It has a fully software-configurable GPU voltage, on a wide range from 0.5V up to the normal 1.1-1.2V, without the need to flash the BIOS. Lower voltage increases performance efficiency.