Just an idea, so... if someone has really crap GFX card, why not resize the textures smaller so he can play those very big maps? would just take a bit longer time to launch the game (or very long time

). though, im not sure how crap it would look after resizing
Wouldnt the fastest way to do it to make mipmaps for each texture square, and then taking the second mipmap of it? or something like that... not so familiar with openGL stuff yet...