As for mounting it, I know you can find brackets to replace the font handles on the G4 towers to fit them in a rack. I can see at my other job (I do computer support and some sound engineering at a recoring studio) if I can find out about brand names or whatever (we have a Quicksilver mounted that way).

Hard drives: the hard drive bus is just a Ultra ATA/66. You should be fine.

Optical drive: don't know much about it, but I'd imagine you could make it work.

Noise: most G4 towers (the Mirrored Drive Doors being the most obvious exception) are PRETTY quiet. I mean, not as quiet as my G5 at work, but not NEARLY as load as most beige-box PCs.

Riser cards: no idea.

Best of look, it seems like a good, pretty straight-forward project.