FYI Shadows render the entire scene a second time from the view of the lightsource to create a depthmap to use as a shadow map - this pretty much doubles vram use theoretically. I'm unsure what framebuffer does exactly - but I imagine it's a screen space shader manager, or it pushes rendered frames to vram to apply effects on it - this also could use a lot of video memory - I suggest using afterburner or rivatuner to monitor vram usage and troubleshoot. Procedural Background can also be rather memory intensive - and the textures the game use are ALWAYS loaded - depending on the texture pack you're using, this can be a very big vram eater. My old gtx560 (1gb) used to cap out vram if i enabled procedural backgrounds @4096 resolution alone.