or the GameXStream wasn't really putting out the 700w, ProXStreams are good OCZ psus, the others (GameXStream, ModXStream, etc.) can be iffy, that 700w GameXStream is also not on AMD's list of approved psus for that card while some 550w and 650w psus are, there are also plenty of 750w psus on the list as well, the key is getting a quality unit, I would trust a Corsair HX620 or TX650 to run that card more than a Rosewill or Logisys 1000w psu...
from just about every review out there the power draw for a whole system with a 4870x2 is under 500w, I saw one extreme system where the power draw was 680w+, but that was with a Quad Core Extreme cpu, three Raptors, yada, yada, yada, but that is the highest I have seen and seems to be the one exception to the norm, your system's power draw would definitely fall right around the norm imo and the 5850 requires considerably less power than the 4870x2 (~66% or 2/3)...