Gigabyte's Socket 939 K8NXP-SLI motherboard is based on NVIDIA's nForce 4 SLI chipset and comes with a very well-rounded feature set. It offers two x1 PCIe slots, two x16 PCIe slots for SLI operation as well as two normal PCI slots and lots of accessories. In order to activate the "classic" SLI mode, i.e. using both x16 slots, a small circuit board that sits between the slots has to be plugged in correctly. The bandwidth of each of the slots is reduced to x8 PCIe in SLI mode.
When using the 3D1, which only takes up one slot, the SLI motherboard needs to be set to normal mode. The motherboard's BIOS then automatically recognizes the card when the computer is switched on, changing the x16 PCIe slot over to the required dual x8 mode.
A look at the Gigabyte K8NXP-SLI.