I saw a review of the GeForce GTX 460 graphics card. It has 56 texture units. I'm not that knowledgeable about graphics effects. But, the ones I know use around 3 or 4 texture units. In this graphics card case, that would leave a lot of texture units idle. How are these graphics cards with so many texture units used?
56 texture units means that approximately 56 texture filtering operations can occur in parallel in a single clock (throughput) (some operations take longer than others - aniso footprint size, texel size, etc.). The hardware will utilize this automatically since it processes many pixels in parallel. The number of texure slots used per shader does not affect this.
@FuzzySpoon: That isn't the case. Even a single cube will be 56x times faster than a hypothetical 1x texture card, since it is parallelized at the pixel (actually pixel texture request) level. This assumes of course that the bottleneck is texture filtering in both cases.