When I run a OpenGL program, and switch to fullscreen, all the textures disapear. The other way around has the same effect, from fullscreen to windowed mode.
I have a Viper330 (Riva128, 4Mb) and I use SDL to create a glx context. The Xwindows is in 800x600, as otherwise it doesn't have any memory left to create the context.
This is not a SDL bug as it works under windows (same video card) and both linux and windows work correct with a mga200 card.