GeForce 256 Fill Rate
When NVIDIA, 3dfx, and S3 list fill rate numbers, the numbers are based on the rendering pipeline, the number of pixels of texels the engine can pump out each clock cycle. Let's say a graphics chip has a dual pixel pipeline. If the chip runs at 125MHz, then the manufacturer will list the pixel fill rate as 250Mpixels per second, because 125MHz x 2 pixels/clock = 250 million pixels/second. The TNT2 Ultra has a 150MHz engine clock and has a dual pixel pipeline for a 300Mpixels/s fill rate. The 3dfx V3 3500 has a 183MHz clock, but only pumps out 183Mpixels/s because the V3 architecture only has a single pixel pipeline.
Now you're asking, "wait, if the TNT2U has twice the fill rate as the V3 3500 why isn't the TNT2 twice as fast as the V3 in all the Quake 3 benchmarks?" Easy, fill rate is usually measured in pixels/s or texels/s. The texel fill rate is more important in multi-texture FPS games such as Q3 and Unreal because those games often take two or more passes to draw a complete frame depending on the number of graphics options you enable. Being able to do two texels in one pass is the same as being able to do two pixels in one pass if a game requires multi-texturing. While the V3 only has a single pixel pipeline, it can push out two texels per clock. The TNT2 can do twice as many pixels per clock, but it also has the same texel fill rate as the V3 -both cards can do 2 texels/clock. This means that the 150Mhz TNT2U has a 300Mtexels/s fill rate, and the V3500 has a 366Mtexels/s fill rate.
The GeForce 256 Pipeline
The GeForce 256 claims to have a "Four-pixel rendering pipeline," which means that it can push out 4 pixels per clock cycle. According to 3dfx CTO Scott Sellers, "It appears as if the raster engine in the GeForce 256 is really just 2 TNT2 raster engines put in parallel (with some feature additions added)." Yes, the GeForce's rending pipeline does seem to be two TNT2s cobbled together. As with the original TNT2 cards, the GeForce's texel fill rate will be the same as the pixel fill rate.
The GeForce 256 has a relatively low clock speed, 120Mhz, but that's not surprising considering that the chip has over 23 million transistors -more than the Pentium 3 or Athlon. With that many transistors, heat will certainly be a problem and that's why NVIDIA has to keep the clock speeds low. With a 120MHz clock, the GeForce has a 480Mpixels/s fill rate which translates into a 480Mtexel/s fill rate.