A Voodoo 3 would be your best bet. The V3 is less CPU dependent - where as the K6-2 is not very FPU efficient, the V3 will make up for the K6-2's loss.
I have a ATI Rage Fury 128 with a K6-2/400.
The ATI is a beautiful card. The colors are great and vivid, but the Rage Fury is a little CPU dependent meaning I see FPS decreases in high geometrical areas in some games. Hence, an ATI would be a great card with any type of CPU.
The worst card to get is a TNT2 w/ a K6-2. The TNT2 is very CPU hungry. It needs a fairly powerful CPU to help render. I've also heard many bad stories about the K6-2 / TNT2 combo.
Forget the horror stories mentioned above. Here are some numbers (AMD K6/[email protected], 128MB, Card 1 V3-3000, Card 2 TNT2):
3DMarkMax 2926 3144
Fin. Real. 3.56 3.64
Unreal 800 39.6 28.1
Q2 Crusher 800 23.6 42.6
You see, the cards are nearly equal. In Unreal the V3 ist fantastic. On the other side the card sucks in Quake2. So it simply depends on your favorite game or the price you want to pay. If the TNT2 is cheaper, buy it. If not, take the Voodoo. The drivers are good for both cards.
I'd say get the voodoo 3 2000. I didn't want to suggest any RIVA right now because I was barely lucky enough to make a Riva TNT run on a K-6 2 450mhz. It took quite a lot of time and effort. Now I have a 500mhz AMD and the V3 2k and it works great. For me, if you want performance perhaps you could get the RIVA TNT2 but I don't suggest it. If you want compatibility get the voodoo 3. I have tested the voodoo 3 2000 on all computers here in the house and it haven't failed a bit. Given motherboards are: Soyo 5EHM, ASUS P5A-B, Redfox Ali, Freetech Slot A. Where as the RIVA TNT only worked in the P5A-B and the Athlon Mobo. Hope This helps