Yes you can use the 660, no you cannot sli it with the apu graphics. This assumes you have enough psu to run the 660.
The 660 would be doing all the graphics chores, and you would be hooking your monitor up to the video card output rather than the mobo output.
You would get a significant increase from this. Moreso if you went 660ti/670/760/770.
Need more info on your system if you want more specific answers.
Certain AMD cards will crossfire with your apu, however it is debatable whether you would gain performance, since not all games support crossfire.