650 would be sufficient for sli 660's but 750 will give you a bit of breathing room.
A single 660 will be plenty for 1080 resolution.. if you think you'll need more power or will go SLI for a larger or multi-monitor set-up in the future then I would say grab a 670 now instead..
or if it's going to be over a year away then your best to probably get the next generation of card and stay on a single card.
Galaxy GeForce GTX 670 2GB Video Card (67NPH6DV5ZVX) - PCPartPicker
Stock cooler (though I think a 680's stock cooler) but a pretty good deal..