2 x GTX 660 = roughly 1.3 GTX 680.
3 x GTX 660 = roughly 2.1 GTX 680.
The build looks fine but in the future you might want to buy at least one more 660 for better performance. Also, buy an SSD sometime down the road. They're pricey but they really are worth it. The prices on them are falling pretty quick too - for example OCZ was the first to launch a PCIe 1TB SSD and it retailed for $2700. The price has already dropped to less than half that. In a few months they'll be as cheap as HDD's are now.
It should be but I can't be entirely certain. As a rule of thumb I'll never buy a case online because I'm never going to be 100% sure if everything will fit inside. You're better off ordering all the parts and building a pseudo PC without a case, measuring it, then going to a local retailer with those measurements. Might look a little funny examining cases in the store with your tape measure, but it would be the smart thing to do.