Voodoo products offer the best optimized video cards right now for K6-2 units. It's all in the drivers. If you get a Voodoo II remember that your highest resolution will be 800 x 600. (Unless you buy two cards and run them together with 3dfs'x SLI setup). I've run a TNT and dual Voodoo II's and they worked great. But they do work best with games written in Glide (the game box will show the 3dfx logo on the outside if it can use Glide). I've found that Glide, optimized for 3D Now, is the fastest video for my K6-2.
On an AMD system, definately stick with a V3. Pick your flavor based on the features you need (TV Out, LCD etc). The card OC's so don't worry too much about the speed of a 2000 vs. 3000 since you can easily run a 2000 at 166MHz.