Direct3D9 and lower report the capabilities offered by the gpu e.g. whether the gpu offers multitexturing, shaders and things like that. Games check these capabilities and due to that a Riva TNT would be able to run basic Direct3D9 apps as long as they don't use shaders. Further the OpenGL version itself isn't that important it is mostly about the offered OpenGL extensions. If you want to unlock all Direct3D7 features you need a Geforce1/2/2mx/4mx or radeon 7xxx, for fully Direct3D8.1 you would in theory need a Geforce3/4ti / Radeon8500/9000/9100/9200/9300 but we don't support pixel shaders on those cards because the functionality is very limited on those cards and is only exposed using some old pixel shader extensions. For Direct3D9 SM2.0 support you need a Radeon >=9500 or a GeforceFX but for SM3.0 you need a Radeon >= X1300 or a Geforce >= 6200.
