Corsair hx850.

An 850W PSU (especially a decent 'brand-name' PSU such as Corsair) should be more than sufficient to run two 5870s.

800 watts is more than enough!

yes it would power it !

though depending on what else your running in your system you might want to head up to a 1000 but an 850 is fine! especially from corsair, a 750 could probably even run it.

If your doing a bunch of loops of water and running multiple HD's in raid then maybe you might wanna step up to a 1000 but i think your 850 is good enough to go

Even a 750w from a brand like Corsair would do, but get 850w; overkill is always nice and you'll always have enough power for lots of fans and other hardware.
A 1000w is way too much though, I am yet to see a desktop PC that comes even close to consuming 1000w.

seriously the 5870 does around 420 watts peak load with NO oc. Stock I7 does around 130 watts? So you are seeing around under 600 with NO oc. Oc it and it will rise around under 750. Since yours is quality, that psu can do more then what it is recommended.

Conclusion: Over kill and ready to go!

420W sounds like total system draw for a 5870 + quad core system. The 5870 draws about 186W maximum, without overclocking.