I have the same card as you, with an i5 3570k. My settings right now are 1080p, DX11 mode (with D3D11MultithreadedRendering="1"), 4x MSAA, Enhanced Alpha To Coverage, SDAO, High Texture quality, High Ambient Lighting quality, High Shadow quality, Medium Post FX quality, Very High Geometry quality, Very High Vegetation quality, High Terrain quality, Very High Water quality, and High Environment quality.
Game runs pretty smooth in the first quest area, usually around 50 - 60 FPS with some spots being higher or lower as expected. Let me know if your experience is substantially different, I'm curious why this game seems to be performing so erratically across different hardware configurations.