SLI stands for Scalable Link Interface (back in the 3Dfx Voodoo days it was Scan-Line Interleave). This is used mainly for gaming as it basically uses the multiple cards to render each frame of video, thus combining the 'horsepower' to speed up frame rates. (note: this is an extremely simple explanation...there is a lot more to it, which you can read about
here.
In short, it will not help your wife very much at all (unless she's a gamer). If you want to help her more in the graphic design area, some things you want to look for are:
1. A good amount of RAM. Depending on the work she wants to do, you want at
least 1 GB of RAM. When it comes to RAM, more is usually better.
2. A large monitor or even two monitors. One thing you can never have enough of when doing graphics work is monitor space. Most current video cards have dual monitor connections on them. So you wouldn't need two video cards to accomplish this.
To answer your other question:
667 is better....well, faster.