I have never had a problem disconecting the monitor while the computer is on, but that does not mean it is a good idea. Plus you really don't have to.
Just setup the G3 to fileshare each drive, once things are setup turn the computer off. those sharring settings will return as you set them even if the moniter is no longer attatched.
Then move the monitor to the new G4 and connect.
If you are using a mixture of OS9 and OSX then you will need to make sure both computers are setup to use either TCP/IP or appletalk. If the OS9 machine is setup with appletalk, then the default OSX computer will not see it unless appletalk is turned on in the network preference panel.
Just setup the G3 to fileshare each drive, once things are setup turn the computer off. those sharring settings will return as you set them even if the moniter is no longer attatched.
Then move the monitor to the new G4 and connect.
If you are using a mixture of OS9 and OSX then you will need to make sure both computers are setup to use either TCP/IP or appletalk. If the OS9 machine is setup with appletalk, then the default OSX computer will not see it unless appletalk is turned on in the network preference panel.