Honestly I don't know Haswell and GTX 700 series power requirements so well, but I think something like a Corsair TX650 would probably be good enough. Remember to check out how many amps you need on the 12V rail. Amps are more important than watts. A good tool would be checking the Nvidia site (I've done this for you, this time, they recommend at least 600W and 42 Amps on the 12V rail) and seeing the power requirements there and then using a power calculator (google PSU Calculator) and getting a good idea of your actual requirements. FWIW The Corsair TX650 I recommended has 53 Amps on the 12V rail. Good enough for one card, but not for SLI. So if your going to do it, might want to look into some really high end powers supplies. Like the Corsair HX1050 or better. Lastly, keep in mind my knowledge is a little dated. If this thread is kept alive, I am sure someone can help more than I can.
I hope it helps!