The only thing the 6850 can do well is tessellation and even then it only wins by 1-2 FPS
http://www.vortez.co.uk/articles_pages/asus_eah6850_directcu_review,20.html
Look at those results, a 1GHz clocked HD6850 and an overclocked HD5850...
With newer games coming and the fact the 6850 is a great clocker its a clear winner over the redundant 5850.