If your current system can handle it then it will be faster than your 660 in games were SLI is taken advantage of. If not the GTX 660 is not too far behind the GTX 580. Be advised it will use more power than two GTX660s in SLI. If you can wait until some price drops on the 780 then that might be a better alternative. The last dual GPU card on one PCB I had, 4870X2, was hot, loud, and had several driver issues in some games. What are the rest of your system specs?
AMD FX8120|Gigabyte GA78LMT Modded (Rev 5.0)|XFX R9 380 4GB|8GB Supertalent DDR3|840 EVO|Raijentek STRYX|Enermax TriathlorEco 650|M-Audio BX8a Deluxe