At stock speeds you can see the AMD HD 7850 is still competitive with the cards around, though a little off the pace in gaming terms against the Nvidia cards.
Once you start overclocking however the difference is much less marked. Indeed the AMD Radeon HD 7850 starts to have more in common with the much pricier Nvidia GTX 570.
DirectX 11 tessellation performance
OpenGL tessellation performance
DirectX 11 gaming performance