How can I disable AGP Texture Acceleration in dxdiag?

I'm having crash problems with Fallout 3, I've tried allot of fixes, now I want to try this one, all details at link below. The substance is, you disable AGP Texture Acceleration in the Dxdiag tool, but as with other posts in the link below, I have no option in the tool to enable/disable the feature. Any help greatly appreciated, I'd love to actually play the game.