This build will probably never consume any more than 350-375W while gaming, so a good 450W PSU would be enough.
With one of these 280X's under full load, their PSU pulled 352W from the wall outlet. Their CPU was idling, so I have to add up to 84W for the i5-4570. Adding that to the 352W comes out to be 436W. So, if the PSU were 90% efficient while pulling 436W from the wall outlet, then that would mean the computer is pulling 392W from the PSU. The gaming power draw will be even less, so that's where the 350-375W estimation comes in. So, having a good 450W PSU would maximize the computer's average efficiency.