SLI will not help with CAD, and is not worth it.
The difference between the 650i and 680i, is that the 680i supports the full x16 SLI mode, while the 650i supports x8 SLI mode. But reviews, have shown the 650i to perform just as well as the 680i. The 680i, also just has more features, such as more USB, gigabit, and audio ports.