Got an old ATI Radeon HD 2400 PRO graphics card here and one day I just googled for it and found out that the latest 2012 driver allows to update default GL2.0 version to GL3.3 and shading language to 3 (or 4 nevermind). I assumed in the beginning of it how awesome it was, but it turned out the opposite! All the GL3 features now really work, but it has got massive slow down to it! I can't even call it a slideshow! Guess what? The software renderer became slow as hell either. This driver update affects the same way gzdoom and skulltag. All the rest games like Battlefield 2, CS Source and others are totally ok with that. CS Source automatically changed my graphics settings to higher.
So if you're still willing to have GL3 and GL4 effects then it's better to go and get a second hand gl3 or 4 card. Otherwise stay with GL2 and use older GZDoom versions.