Basically, the GTX660 will give you a performance edge you can actually see, since it doesn't restrict framerates below 60fps. You can read about it in detail at
http://www.hardocp.com/article/201 [...] y_review/3.
"With Adaptive VSync turned on, the feeling of the game felt smoother compared to regular VSync turned on. The performance felt much like the game felt with VSync turned off. This is the kind of technology we like to see which has improved the innate nature of the gameplay experience. If all you need is 60 FPS in a game for it to be playable, then why not just go ahead and cap the game there so it doesn't exceed your refresh rate. Then, if the game has to fall below that, allow the game to perform at its real-time actual framerate, and Adaptive VSync allows that.
It really is the best of all worlds, with no drawbacks. We didn't find any negatives to using Adaptive VSync, and we tried it out in a handful of games."
You can just enable adaptive v-sync and then forget about it - she'll never get tearing or artificially restricted framerates.