Personally I am for people learning the hard way. I say buy up every card you can and see what happens. If a site dedicated to 6400's cannot answer your question back when it was relevant to many users, people here cannot help you.
Using google and asking around here you have seen what was done in the past. You also need to understand that most of what was mentioned on that 6400 website was done back when the 6400 had considerable value back in 2000 (newer more capable machines were very costly). Most sane people do not spend lots of money on a system when a newer one would be better for the task at hand, and cheaper as well. The video cards mentioned are cheap enough to buy and mess around with these days, so go ahead and see what you can do with them.
The only card I ever used was a cheap ATI 4MB in my 6400, nothing special but it worked. Since I have all the upgrades outside of a G3 (video input, video out, tv tuner) and they need the built in video to operate I decided to just put a 6500 motherboard in and live with it.