as far as the case goes, there are several good deals on g5 cases now on ebay, look here[

as for the linux thing, try my favorite distro:
Macbook 2.1 core duo 1gig 160gb sata
iMac g5 w/1 gig 160gig all-in-one and SWEET!
iBook G4 IGhz
Quicksilver Dual 800's
iPod touch
"Guys, I think we just lost the platform here" Pete Conrad
When in doubt, try SCE to AUX