well a 6850 has MIN wattage of 500 so yes u do need a new pcu
GPU manufacturers' figures are always grossly inflated since manufacturers have no way of knowing the quality of someone's PSU and the rest of their system configuration. Lousy PSUs often fail to reliably deliver even half their rated power, sometimes catastrophically/spectularly so.
The GPU's real-world power requirement is closer to 150W and if you throw in 150W for the rest of the PC, you only need 300W of real continuous power so a high-quality 380-450W PSU would get the job done.
I wish GPU manufacturers would actually publish their card's REAL power requirement instead or alongside their PSU rating recommendation so people who want to size their PSU the right way would have the REAL numbers they need to do so.