The main feature of 680i SLI was support for SLI with a full 16 lanes of the PCI Express bus for each card. However, in total 680i SLI actually supplied 46 lanes of PCIe, which would normally have been used for other expansion cards. Instead, though, some cunning motherboard manufacturers added a third graphics slot to take up the remaining eight lanes, although there was never an obvious application for this feature.
Recently though, nVidia unveiled 3-way SLI support for both the 680i and 780i SLI chipsets, which meant these old three-slot motherboards now had some use. All of which begs the question, what does 780i SLI bring to the party? The answer is in two parts; there's support for Intel's new 45nm Penryn processors as well as lots of extra bandwidth for the graphics.
680i SLI uses PCIe 1.1 across the board but 780i SLI uses a mix of PCIe 1.1 and 2.0 so the two main graphics slots each get 16 lanes of PCIe 2.0 which has double the bandwidth of PCIe 1.1 and the third graphics slot has 16 lanes of PCIe 1.1. That's 48 lanes right there but in total the 780i SLI can supply up to 62 lanes of PCIe so the motherboard manufacturer has the option of adding more expansion slots if they should so choose.
How, you may wonder, is this possible? It's a good question and one that is answered by Nvidia's block diagram which shows that it is probably more accurate to call the 780i SLI a chip collection rather than a chipset as it consists of three pieces of silicon, instead of the usual two. There's the 780i SLI SPP (System Platform Processor) which connects the memory and processor and doesn't have a great deal to do with PCI Express. Then there's an Nvidia nForce 200 chip which does the heavy lifting for the three graphics slots and bringing up the rear is the 780i SLI MCP (Media Communications Processor) which adds a x8 PCIe slot and up to four x1 slots.
It so happens that the 780i SLI MCP is a rebadged nForce 570 SLI chip and the 780i SLI SPP shows every sign that it is a 680i SLI that has been modified to link to the new nForce 200 chip.