From NVIDIA's "How to Build a Quad SLI Gaming Rig" guide :
GeForce GTX 590 in a Quad SLI configuration requires a SLI-certified motherboard with an extra PCIE slot between the two double-width cards to ensure both receive enough cooling air. If you're water cooling the cards, you can get by without this extra slot, but for air-based cooling it is a necessity.
The clear air between the two GeForce GTX 590 graphics cards ensures that thermal performance remains within acceptable limits.
Quad SLI certified motherboards that have the correct connector spacing for use with dual GTX 590s are as follows :
ASUS Maximus III Extreme
ASUS P6T7 WS Supercomputer
ASUS P8P67 Deluxe
ASUS P8P67 Pro
ASUS P8P67 WS Revolution
ASUS Rampage III Extreme
ASUS Rampage III Formula
ASUS Sabertooth P67
EVGA P55 E658
EVGA X58 FTW3
MSI X58 Pro-E
With the combined power of four GTX 500 GPUs, Quad SLI can be easily bottlenecked by the central processor. Therefore, it's important in this case to have a fast CPU to get maximum performance. Core i7 CPUs with Turbo Mode enabled are highly recommended, and for even better performance, you may even want to consider overclocking your processor.
Operating a Quad SLI GTX 590 system requires a power supply that has four 8-pin PCI-Express supplementary power connectors, each of which must receive 150 Watts. Because of the complexity of designing, building and ensuring that the power supply meets these necessary requirements, NVIDIA works directly with power supply manufacturers to certify their products for Quad SLI use. The list of power supplies certified for use with a Quad SLI GTX 590 configuration is as follows :
AcBel PC8055-000G 1100W
Nexus RX-1.1K Gold
Thermaltake Toughpower W0171 1500
A good computer case provides the proper airflow for cooling the graphics cards, without sounding like a jet engine. The optimal chassis has multiple fans that draw in cool air and push out hot air, with a large fan positioned on the door side panel to blow cool air over the graphics cards. The case must also support 11 inch graphics cards. As with the power supplies, NVIDIA works with manufacturers to ensure that their cases are certified for use with Quad SLI configurations, and as of today the list of certified cases is as follows :
Coolermaster HAF X
Silverstone Raven RV02
Thermaltake Element V
While the right case and air-cooling setup can result in low temperatures in a Quad SLI PC, water cooling can take thermal performance to the next level, resulting in an even cooler, quieter system that can be heavily overclocked. We've worked with several of the top water cooling companies in the world to build custom water blocks for the GeForce GTX 590, including :
System RAM / Operating System
For the best performance possible in a Quad SLI configuration, 6GB of system memory is highly recommended, and because of this the use of a 64-bit operating system is required. Windows 7 64-bit is recommended, though Windows Vista 64-bit will also suffice.