Your power supply will be fine. People telling you that 750 watts is necessary for 2 x HD6850s have no concept of how little power most computers need.
From Guru3d.
" Our test system is based on a power hungry Core i7 965 / X58 based. This setup is overclocked to 3.75 GHz. Next to that we have energy saving functions disabled for this motherboard and processor (to ensure consistent benchmark results).
On average we are using roughly 50 to 100 Watts more than a standard PC due to higher CPU clock settings, cooling, additional cold cathode lights etc.
With that in mind. Our normal system power consumption is higher than your average system.
Graphics card IDLE WATT FULL WATT
Radeon HD 6850 1024MB 176 278
Radeon HD 6870 1024MB 173 295
Radeon HD 6850 CrossfireX 186 393
Radeon HD 6870 CrossfireX 184 421
http://www.guru3d.com/article/radeon-hd-6850-6870-crossfirex-review/4
Now if you are overclocking everything in your computer to it's max you might want to go with 600-650 watts.
AMD recommends 550 watts for HD6850 Crossfire. That assumes a junk power supply. The OCZ ZS is a very good power supply.
http://www.realhardtechx.com/index_archivos/Page362.htm
So basically if you overclock everything to it's max and run Prime 95 and Furmark 24/7 you might need a more powerful power supply. At normal settings with normal useage you are fine.