afaik the nvidia 5x - 7x wont give you any 3D performance under OS9. so they are only useful if you want to drive a 30".
in addition, the AGP versions of them will have to be prepared to run as 4x AGP instead of x8, you probably know what that means.
a proper solution for a first generation G4 should be the geforce 2 mx, which is found in most quicksilvers. you can easily aquire one by buying an outdated or broken quicksilver 733/867
to your other question: like zef said, you can look at AGP as a kind of "yet another PCI slot". MacOS classic and Mac OSX PPC are very bad in supporting external GPUs, but therefore, as opposed to older windows OS, you can combine any graphics cards in AGP and PCI slots like you wish and use them together or only one of them.
as soon a monitor is turned off or is no connected at all, even the preference setting for "main monitor" will automatically switch, so that you are not left with no menubar.