You can't with an newer ATI card running recent drivers. Their implementation of the updated Open GL screws them up in both games. The best you can do is either turn off framebuffer effects outright as Qliveur suggested, or use Shem's Realistic Visual Effects mods - K1 version
and TSL version
Your only other option is to buy an old card and use the stock drivers that come with it. My old Geforce 6800 ran both games fine. I now have an HD 4850 and both games are plagued with problems.