I would rather look at the 1000 watt power supplies because 700w would be really tight. Two 5870's consume about 600 watts atleast what i've heard of + the 950. : )
And make sure the power supply is compatible with crossfire.
I do not think so looking at the charts from Xbit. They are rated for 120-160w at stock (physical load for each card). The 5830 and the 5850 use the same amount of power and slightly less than my x1900xt. Total system you will be ok but somewhat a little much for your psu so long the rest of the system is efferent. Such as not using a 140w Phenom 2 or a 130w I7 9xx.