OK I got a great reply from the comp.graphics.api.opengl newsgroup:-

GLuint buf;
glGenBuffers(1, &buf);
glBindBuffer(GL_PIXEL_PACK_BUFFER, buf);
glBufferData(GL_PIXEL_PACK_BUFFER, width *...

Hello, is it possible to compress a texture that's already in vram?

I'm creating textures using fractals so I have to render to them uncompressed, I then load them back into CPU memory, and then...