In order to enable the 'Very High' setting you will need a video card that supports DX11 and the latest drivers
Electronic Arts and BioWare have released a high resolution texture pack for Dragon Age 2 enabling higher detail replacements for most textures in game. A big difference will be noticed on level art especially. Using high-resolution textures requires a graphics card with at least 1024MB video memory. Most of the benefits from the Hi-resolution textures will only be seen if you are able to run the game in DirectX 11.