I have NFS: Porsche on my Windows 98SE test system and I have run a LOT of cards in that game for several years. I really wouldn't expect great performance from a K6-II or even K6-III+, but I guess it depends how picky you are about smoothness.
My tester was running a Pentium III 850Mhz for most of its life (now running a 1.1Ghz) and it seems basically sufficient for that game, but that is somewhere around 3 times as fast as a 400-500Mhz K6 2\III (depending on the benchmark). If I remember correctly, on that system even a Voodoo3 3000 is not smooth enough for me at 1024x768. With that config the game seems to suffer from fluctuating frame rates and "surging" (movement speeding up or jumping ahead unexpectedly).
I would consider NFS:Porsche more suited to a high end PIII or Athlon\AthlonXP build with a beefier GPU. At least a Geforce 2 GTS or Geforce 3.
But again, if you aren't real picky and don't mind the frame rate going up and down and your car jumping ahead 20 feet suddenly, you can probably play it on even a Voodoo Banshee and a K6-II. Just expect to turn settings down to make it playable.
If you save NFS:Porsche for a different computer and want a somewhat balanced K6-2\3 system to run games up to 1998 and maybe less heavy games from 1999, a Banshee would be okay but a V3 would allow you to run at higher resolutions with better performance.
EDIT: I found a nice thread all about the performance of NFS: Porsche and this post in particular has some useful benchmark numbers. Pay attention to the average vs low frame rates. This is on a Pentium Dual Core E5800 running at 3.8Ghz... so, zero chance of CPU bottlenecks. Even a Geforce 2 MX 400 has lows in the 20s at 1024x768, which I wouldn't find acceptable.
Now for some blitting from the back buffer.