700 watts is more than enough to power a system with two ATI Radeon HD 5850 or 5870 video cards in Crossfire mode. The official recommendation at the ATI web site is a 600 watt power supply with 40 amps on the 12 volt rail(s) for Crossfire.
That's not really expensive. That's about average for a good PSU in that capacity range. And even so, a 650 watt Corsair or Antec has more usable power (50 - 52 amps @ 12 volts compared to the OCZ's 46 amps).
Ive been looking at this power supply as well, and while the wattage is enough i am fairly certain it cannot support Crossfire for two 5850/5870s due to each video card needing two PCIE connectors, so 4 in total with two cards. and this power supply only has two PCIE connectors... not 100% certain, but am I right?