The hardest part will be getting the 6400 open. This site has an illustrated guide to getting into the 6400:
http://www.zone6400.com/manual/6400manual.html. You'll only need to remove the lower front cover.
1. You can pull the drive from the 6400 and install it in the G3 and copy the data. This link talks about buying a second drive for the G3; ignore that and just use the 6400 drive, the setup is the same:
http://www.xlr8yourmac.com/G3-ZONE/y...IDE/index.html.
2. Get a cheap pci ethernet card for the 6400 then connect the G3 and 6400 with an ethernet cross-over cable. You could also use two standard ethernet cables and a hub. Once you have the file sharing configured you'll be able to copy everything to the G3. I paid about $10 for the card and the cable was $15.
For info on this see "3 Macs and a Printer" site:
http://atpm.com/network/. It explains the setup and file transfering.