It really depends on your entire system, not just the cards. I have a watt meter that measures power draw from my wall.
I don't have two 5870s but my system build should be similar.
i7 930 @ 4ghz
ATI 5970 @ 950/1200
3 sticks of DDR3 1.65v
2 7200 RPM HDD
1 SDD
1 Water Pump
3 140mm fans
3 120mm fans
All this takes 200W idle.
Gaming load goes to 400W.
Prime95 + furmark at the same time load takes 600W.
This is from the wall. My PSU is 80 Plus Silver which is about 85% efficient, so the power I draw from the wall is 15% higher.
This is a more realistic qoute than the specs of each individual components because I overclocked as well. When I don't overclock.
Idle hardly hits 200W, most of the time at 150W-180W
Gaming Load is around 350W.
Prime 95 + furmark load is about 450W.
A lot of people would tell you to get really heavy power supply but most of the time it isn't needed. Its always good to have extra headroom but 1KW is just overkill.
If your not overclocking. A Seasonic 650W with Gold Certification would be plenty.
The powersupply is also modular. I always recommend modular unless you are running dozens of hard drives among other things. Most of the time when someone's powersupply explodes, it isn't because he didn't get a big enough powersupply but rather he didn't get a quality power supply.