DFI NF4 Ultra-D to SLI-D Mod Guide
 

If you have the DFI nForce 4 Ultra-D, you may be surprised to hear that it is actually an NF4 SLI-D board with the SLI part not activated! The board is physically exactly the same apart from one tiny electrical connection on top of the chipset. So all that is stopping you from activating SLI on your supposedly non-SLI board is one tiny electrical connection. Of course, it takes a little work to get to it