No, a 6850 < 5850 < 6870. So you need a 6870 to be more powerful than a 5850 or the less powerful 6850. Don't let the naming scheme confuse you. Look at the benchmarks. http://www.tomshardware.com/reviews/radeon-hd-6870-radeon-hd-6850-barts,2776.html
That power supply is fine for any single graphics card and will run the 5850, 6850 or 6870 in Crossfire.