2 GTX660s should easily outperform a single GTX670, but there are going to be shortcomings with any multi GPU setup.
Specifically, it's more dependant on quality drivers, and will require more power and potentially more case space/cooling.
I run a single GTX660 (The Gainward version), which I chose for its low power use, compact size and competent 1080P gaming credentials. If I'm honest, I'd probably be very tempted by the HD7870XT/LE today if in the same situation, but the GTX660 is still a good value performer.
As for the power question, a bit of it depends on your choice of cards and whether or not you are picking an overclockable processor when the time comes.
A high quality 600W PSU will suffice for a GTX660 SLI setup or single GTX670 (which will be fine on a solid 500W).
If you want to push overclocking, adding a bit of headroom to those figures is perhaps wise, perhaps 100W or so to give yourself room to manoeuvre.