Up to 8 times the texture capacity:

"This table shows the memory requirements for various sizes and color depths of textures
without compression. With FXT1™ texture compression, 32-bit images are reduced by a
ratio of 8:1 without a perceivable loss in image quality." - 3dfx FXT1 Presentation
More bandwidth available to increase fillrate and framerate:
"This chart shows the relationship between texture bandwidth and fill-rate.
Notice the dramatic increase in texture bandwidth needed to achieve a desired
fill-rate (4x as much bandwidth is needed when using bilinear filtering!).
Without enough available bandwidth, fill rate performance suffers." - 3dfx FXT1 Presentation