The best card at the present moment for any K6-2 is the Voodoo2 with 3DNow! drivers. The V2 with the new drivers is almost twice as fast as the generic drivers. However expect the lead to swap when Riva bring out some better drivers for their TNT. Basically if you've got a K6-2, you NEED to use the 3DNow instructions wherever possible. I would hold off buying a TNT until you know for sure that new drivers are coming out, they could be weeks and they could be months. Even if they do come out, they may not be well optimised and a V2 will still beat it.
Naturally, once you're convinced that good drivers are coming out that make use of 3DNow (or you're willing to take a gamble, the odds are in your favour) then I recommend the TNT. The velocity 4400 is currently the fastest TNT board out there, it looks excellent.
Disclaimer: this is all based on what I have read on about 19571957294 different internet pages. My computer is equivalent to a P90 so I didn't even bother with a 3D card for it. All that will change in december, that's new computer time