A system with a single HD6850 requires a 450W PSU with one 75W 6-pin PCIe power connector and 28A on the 12V rails.
Do not cheap out on the PSU; it is the heart of your PC. My signature below has a good article on quality PSUs. In general stick with a reputable mfr like Seasonic, Antec, Corsair, Enermax, PC Power and Cooling. Where are you buying your PSU? What's your budget? Do you have a link to a website where we can make recommendations?