I think the information that you’re looking for (but hasn’t been spelled out here,) is that when people talk about using multiple GPUs on one computer, they do so by having the GPUs in multiple PCIe slots, and send compute jobs to them individually. They’re not using SLI. It would be great if one could link 2-3 cards with SLI and have them work as one big unit for CUDA. But AFAIK, not an option at this point in time. (Hint, hint, nVidia people, …)
Some issues to be aware of:
Each card needs 1-2 PCIe power connectors, of either the 6 or 8 pin type; the 6+2 style can drive either, and give you more flexibility with moving cards around between different machines. And each card needs a decent amount of power, a function of the card. Your power supply needs to have enough power on each PCIe power line - make sure that you aren’t in a position where you have enough total power, but multiple 12V nodes can’t power all of your cards at once. And as has been stated here a number of times, have some overhead. Power bottlenecks can produce issues in other parts of the system that are time consuming to diagnose. Spend a little more here and avoid these pitfalls.
Each card needs a PCIe slot on the motherboard (double width,) preferably with x16 bandwidth, but if you’re going for more than two, this won’t be possible for all of them. If you don’t send a lot of data back and forth from the CPU to the GPU, you can probably get away with x8 or lower (some users here report that x1 is sufficient for certain operations, and x4 is more than enough.)
If you’re using the latest and greatest 400 series cards, they run hot, so it’s nice to have some space between cards so the side fans on the card can draw air in. Pressing one fan against the back of another card reduces airflow, possibly shortening the working life of the card (but we don’t have much data on that as the 400 series cards have only been in service for ~ 6 mos.), and perhaps even reducing the stability/reliability of your calculations. But if you plan to put 4 cards into one machine, you’ll need a special case and motherboard, or have to cram them all against each other.
Also, if you’re just using one machine, you’ll need either video out on the motherboard, or take one slot and dedicate it to an output card. The MSI 980i-G65 motherboard has nVidia video out and can take 3 PCIe cards at x16/x8/x8, or two at x16/x16 (but they are all right next to each other.)
Finally, you’ll want a case that has good airflow. The Antec Three Hundred Illusion runs about $65 at NewEgg, and comes with 4 fans, including a top exhaust fan, with a spot for a side intake fan that blows right on where the graphics cards sit. Tom’s hardware consistently rates it as a great budget choice for a gaming case, with no frills, good airflow and washable air filters. For machines with multiple 470’s or higher, the Antec 902, Cooler Master Storm Scout, or Silverstone Raven RV02 have good cooling, to name a few.
Finally, CUDA computers share a lot of the same issues as high end gaming cases, so using their ideas as a base is a solid way to approach the issue. Check out this build: http://www.tomshardware.com/reviews/newegg…dware,2753.html , as well as the associated articles. But note that the CPU is less important in CUDA calculations that aren’t bandwidth limited, so the AMD solutions might be more cost effective, but that depends on your problem.