What I mean is: If you want to put any upgrade over 500 MHz, make absolutely sure it has an AGP slot.
As for the case thing, you can replace anything in it you want. I don't recommend chaning the motherboard unless it breaks though. And if you do change motherboards, you need to get a hold of the port shield for that motherboard. Also, the motherboard you are buying must have the exact same # of AGP&PCI slots
EDIT: Any 450 or 500 MHz G4's are AGP models (that is, unless the processor has been upgraded...)
[ February 26, 2003: Message edited by: The Muffin Man ]