On second thoughts, debugging by eye - so highly unlikely to be right but the line
gl_FragColor = clamp(sum*sum*0.04 + texture2D(bgl_RenderedTexture, texcoord),0.0,1.0);\
looks unlikely to be right to me. How many components does your texture have? I guess 4 as you are loading PNGs, so this line would try to stuff 6 components into into a vec4 and fail I guess.
ATi drivers have always been less forgiving of type conversions than Nvidia ones.