A standard HD6850 requires a minimum 500W PSU. The one you intend to buy is sufficiently factory-overclocked as to need a second PCIE power connector, so 550W to 600W is probably a safer range. Antec, Seasonic, Corsair, Enermax, and XFX are among the better brands with models in that range.
For games, a higher clocked Phenom II X4 may provide better performance than the X6; and an Intel Sandy Bridge CPU (even the locked 2400) will be stronger still. If needed to make budget, drop down to a non-toxic HD6850 (you can OC it yourself) or 1GB GTX460.