Aliens vs Predator uses many DirectX 11 features for cards that can support them. This includes tessellation, like the other DX11 games tested, and other lighting and anti-aliasing effects. With all settings maxed out, the 7850 performs similarly to the older 6870. They're both on the cusp of offering playably smooth framerates, but things get rougher on some of the more intensive scenes.
Disabling MSAA and makes AvP playable on the Radeon HD 7850. It takes the lead by a decent margin, and it seems like AA dealt it a big performance hit compared to the other GPUs.
S.T.A.L.K.E.R. Call of Pripyat uses the XRAY 1.6 game engine which offers a wide range of DirectX support, version 8 through 11. It does support tessellation and its implementation results in a lot of realistic looking buildings and scenery. And to help create a scary ambiance, lots of dynamic lighting effects are used. Maxing out all of the settings, the 7850 took the lead and beat out the last generation midrange cards.