It would be simple enough to cover the opening for the optical drive, and it would save some weight. Alternatively there are kits for adding a second HD (which would then be software RAID'ed if need be) where the optical drive should be. I like the idea of it looking like there is a slot, but you can't put anything in it due to something being behind it. I also like the idea of no one knowing I had a RAID between my legs
A cool sounding project for sure, but if I may point out the major flaws for me:
1. Leopard and 1.25GB RAM won't get you too far, especially on a slow processor like the G4.
2. No 802.11n card means not having an optical drive is more of an issue (although there is gigabit ethernet in these machines so you trump it there).
3. Going from PPC to Intel was a big change, but going back again is worthy of suicide.
4. 1024*768... 'nuff said.
5. The display is just so darned dull.
I guess that's a little picky, but I wouldn't do it now. Still, I'd probably give it a go as a project given the cash. Nice idea.