Look for dual socket 1366 motherboards: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010200302%201071346469&bop=And&ShowDeactivatedMark=False&ActiveSearchResult=True&Order=RATING
Make sure that you carefully check and understand the specifications, e.g., the motherboard size (Extended ATX or SSI EEB) to ensure that your workstation/server case can accept it without modifications, the number of memory slots, the number and type of PCIe slots, PSU requirements, etc. I would also recommend buying ECC server memory modules for optimal stability.
If you require more than 2 Intel Xeon CPUs (Xeon 7400 Series), you'll have to get a server or workstation from HP, Dell or IBM. Newegg.com sell a few quad socket Opteron motherboards, but they don't seem to support the latest Opteron processors, making them uncompetitive against dual Nehalem motherboards. Personnally I would consider buying a workstation instead of building it myself, but that's a personal preference. Building a system that matches the specs/performance of an HP Z800 Workstation isn't necessarily easy.