Resolution: leave as default (the game will detect the proper resolution).
Display Mode: Fullscreen.
V-sync: enabled. If you disable v-sync, you might be able to move the camera faster but it will feel weird.
Frame Cap: leave it to 60.
Scroll down.
Model Quality: High.
Texture Quality: Medium.
Texture Filtering: Trilinear.
Shadows: enabled.
Light Shadows: disabled.
Shadow Quality: Very low.
Advanced
Anti-Aliasing: None.
All the other options from under Anti-Aliasing: disabled (Ambient Occlusion, Screen Space Reflections, Depth of Field, Motion Blur, God Rays, Bloom).
The options that are consuming the most resources are “Shadows” and “Anti-Aliasing”.
The game still looks decent with these options disabled. Try first to set the shadows to very low. If the camera moves decent, you can play the game like this. If it lags too much, just disable the option.
The “Fake Fullscreen” option instead of “Fullscreen”, doesn’t do anything. The game actually runs even worse.
For what is offering, the game is poorly optimized but it looks and runs much better compared to the early alpha versions.
I play with the mentioned settings and the game runs smooth and perfectly playable. This is my system: Intel I3 3GHz, 8 gigabytes RAM, ATI Radeon HD 5670 (1 GB RAM).
If you run with a 2 GB RAM videocard, the game should run perfect. Original Sin is just requiring a lot of RAM from the video card.
Last tip. You get better framerate if you play with the zoom in, a bit. If you have full zoom-out, the game will require more video card RAM and will obviously drop some framerate.
How is the game working out for you?