sorry, forgot the "only", my mistake.
2x GTX 670 are better than 2x GTX 660 because the GTX 670 is faster than a GTX 660.
If you need more graphics power, adding a second GTX 660 will boost your system a lot. So yes, this will be a nice upgrade.
But keep in mind that some games or applications don't support multi-GPU rendering, so they will use only a GTX 660 to work. In this case, a single fast GPU should be the best choice.
so, what do you need? what applications are you using?