I had a K6-2 @400mhz and upgraded my voodoo1 to a Geforce (which is much faster than a PCI Voodoo3). My Quake III framerates went from 18 to about only 22, maybe 24. Of course that's at 640x480, 16-bit color because that's all the Voodoo could do. With the Geforce I could do 1024x768x32 at the same 22 frames per second. The framerates were the same at all resolutions (all with mimimal video quality settings) because the K6-2 was the limiting factor.
The MX-400 will be just as limited for the same reason. You will gain 32-bit color though. If you have a faster K6-2 then you will do a little better but you will run into the same problem. It's too slow compared to the video card.
By the way, I later upgraded to a Duron system with the same Geforce card and now I get 57 fps at 1024x768x32 but with maximum quality setting. That old 640x480x16 test that only reached 22 fps is now hitting 118 fps.
What I am trying to say is a fast video card is going to do very little if the system is too slow. Upgrade the system and your voodoo3 will be able to reach it's full potential. If that's not good enough, then upgrade the video card later, too, but the card alone won't do much.