2x GTX 660 would be the most powerful solution. But to SLI there is no point in getting overclocked/superclocked cards as SLI will force them to run at reference speed.
Last time I heard was that they must both run at same speeds not refrence... But I might be wrong...
2x660 all the way but 670 if your upgrading in future. Note that new Nvidia GPUs are comming this month