I don't play Quake, so I can't give you numbers, but I do have a K6-3 400 system that I recently put an Asus V7700 deluxe GF2 in. I like this setup because it allows me to run new 3D stuff better, and it will still be a damn good video card for when I upgrade to a DDR athlon system. IT has worked nicely for me. I am not too sure about how big an improvement you will get, but it should be similar considering that our processors are similar.
"Water-Cooled CPU Runner"