Assuming the Preview Release detects your onboard ethernet it shouldn't be too hard to get online. I managed it on my 7500. The main problem with trying to use BeOS as a main OS is the lack of software (but then Rhapsody would have fared little better as it to had a short shelf-life). 4.5 and up would definitely be the way to go but if you want to try some apps in the Preview Release check out the obsolete folder on this FTP. Anything labeled for AA (Advanced Access) should work on the PR as they were both DR9 (in fact I don't think there was much/any difference between the two but I haven't seen a copy of AA to verify this).

You'll need to set up the connection manually as I don't think DHCP is supported. I'm working from memory here but you go into the Network preferences panel, click on add network interface and choose yours (onboard powermac most likely). Assign a manual IP to the machine, add the gateway/router IP (the IP of your router) and dns servers. While you're there you can give the machine a network name. You could also enable FTP and/or Telnet access (I find FTP is handy for getting files in and out). Then click on restart networking. If all goes to plan, you can fire up NetPositive and get a very broken view of the modern innernet.

As for sound that should work out of the box. But your 6500 is in the "unsupported but Compatible" section of the PPC HCL along with my 5400.