According to this review (very reputable source) A pair of 6850's in crossfire draw 245w. http://www.guru3d.com/article/radeon-hd-6850-6870-cross... Your CPU will draw a max of 95w. let's say the HDD's DVD's and fans use another 50w, which is quite generous. you have a total of 390w. Now we need the amperage, right? assuming all of that stuff is drawing off the 12v rail (which it's NOT) you'd still only be drawing 32.5 amps from the 12v.
The 6850 is a very efficient card, drawing only around 12 amps at full load. You should have plenty of headroom. You can use this PSU calculator (regarded as the most accurate) to puzzle out the numbers yourself. http://extreme.outervision.com/psucalculatorlite.jsp