If you have 560 now then no need to upgrade to 660 or no card now. I wouldnt do that if i were you.
If you really want to upgrade than go higher than 660, like gtx 670 or HD7970.
But in my personal opinion, wait for crysis 3 released. It will require a lots of gpu power. Then upgrade to a crysis 3 recommended gpu or new AMD gpu if they released.