it might also help if you could find a way to keep the texture in the vid cards memory, that way you wont incure the performance hit of reading back from the frame buffer (thats assuminng you need to adjust the image on teh fly in real time), however I dont think all cards support this (I know Nvidia ones do via an extension)

It is possible to "prioritize" the textures in OpenGL, ie ask OpenGL that some textures may be resident, and others are less important.