It depends on the series. PLG quickly SKG between the GTX 590 and HD 6990.
Almost balanced.
For the price at BWH 1 jt LBH Amd Radeon good. (Ati rename dah). Price / performance better than vga nvidia LBH. If above 1 new jt comparison between his type.
For game times SKG PLH HD 5670 or HD 6670. Or from nvidia GT 440 which is an OC version of the GT 430. From nvidia and amd there are advantages and disadvantages masing2.