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NVIDIA : TMU is an obsolete term, by the way. It's a leftover from when the computation (blend stages) was tied directly to the texture read ability (texture stages). This is no longer true, since the TNT architecture. Modern hardware has texture loading separated from a processor that allows calculation and blending of texture addresses and interpolated and evaluated colors. Very soon, there will be powerful pixel processors where the number of textures that can be read is independent of the number of instructions that can be part of the pixel program. GeForce3 is the first of these processors. Most software developers do not wish the hardware developers to accept a high level shading model and handle multipass for them. The software developers want to program this themselves. Given that there are 10's of millions of TNT2-class graphics cards out there as an installed base, virtually ALL mass-market titles need to support rendering to 2 texture hardware, with multipass. Most physically based lighting models (such as BRDFs - bi-directional reflectance distribution functions) decompose cleanly into pairs of textures per pass. GeForce3, which supports 4 textures in a single pass, is a simple acceleration from that: every 2 passes can be collapsed into 1. For that reason, I believe that Radeon choosing to support 3 textures is an odd choice.
