eden2012 :
Didnt mean to chose you as my solution. And no. I will NEVER SLI. Its a dumb idea. I heard enough micro stuttering stories...
You can unselect it if you didn't mean to choose a solution(or someone else chose it... pretty annoying when they do that in my threads).
Anyway, SLI is a good option if you have the compatible power supply and motherboard(Which I'm guessing you don't, as you planned not to do SLI). As for the microstuttering, AFAIK the issue was fixed by NVIDIA. just get the latest drivers for your 780.
To check for a bottleneck in case of a CPU, just lower the quality to the minimum possible settings graphics wise(such as resolution, texture filtering, AA, AF, you get the idea). That is the maximum FPS your CPU can handle. Depending on the game, if it is very GPU intensive, the FPS may reduce when you increase the settings again.
If the FPS you're getting is acceptable to you, get the new card. Otherwise, first upgrade your CPU. The 660 should last you for such a time as you're ready to upgrade your GPU.