From my mining experience, a 290 (and a 390) can easily push 250w+ if you're @stock volts/clocks.

Put 6 of them into a rig with minimal voltage / clock tuning and you'll be looking at a 1500w pull real fast.

The 2x Antec HCP 1000w suggestion is a good one.  I went with Silverstone 1500w units, which performed very well given the environment.