The HD 5770 and HD 6770 uses the same graphics core. Compared to nVidia cards the HD 6770 falls between the GTS 450 and the GTX 550 in terms of performance. The GTX 460 competes against the HD 6790 and in some cases the HD 5830.http://www.techpowerup.com/reviews/AMD/HD_6790/5.html
AMD does in fact rule the low end to mainstream of the market for better price/performance ratio and the fact that their Radeons use less power. For example, if you only have a 300w PSU, then the best card to buy would be the HD 6670. That card fall between the GT 440 and GTS 450; both of which uses more power than the HD 6670.