A 750W PSU from Corsair/ Seasonic/ Silverstone/ PCP&S would be able to handle that setup well...
The HD 5870 has similar power requirements of the HD 4890...
Here at TOM's, there was an article about a Cyberpower dragon rig, that has an overclocked X4 955 to 3.6GHz + 2x HD 4890 in crossfire, all powered by the Corsair 650TX...And it ran well without any power issues...
So the 750W should be a safe bet for the overclocked i7, HD 5870 crossfire...
And dont overshoot the power requirements...maybe at the max get a 850W PSU from any of the above manufacturers...