Re: 780i and x38 (or p35) chipest
The 780i supports 3-way SLI like cryman said, 650i supports SLI but splits it's 16 PCI-E lanesbetween the two graphics cards so each GPU can only use 8 lanes. The 680i supports SLI but each GPU gets the proper 16 lanes each. Intel chipsets don't support SLI but ATI Crossfire. The X38 and P35 are almost always a better choice than an Nvidia chipset unless you need SLI support as they can reach a higher FSB speed and are generally better for overclocking.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.