The FW400 MDDs are great for OS 9. I bought my dual 1.25GHz MDD G4 new specifically because it could still boot 9.2.2. Later I put a 1.8GHz Sonnet upgrade in it, and it's still my main Mac OS 9 development system. It also still sounds like a windtunnel.
I was able to find a decent high CFM main system fan that... still sounds like a wind tunnel, but only a little.
I remember my Dad had the first G5 with the new thermal architecture, with everything blowing through the front cheese grater grill. That was much better for the components, and blissfully quiet at idle at least.