If the silence is important. I would avoid MDDs G4s.

The FW800 MDD G4 I have (1.42 GHz) is damn loud. Further, the power supply of it may not last for long.

I know you said you already own a LCD display, however, an iMac G4 (up to 800MHz) might be your better choice for the noise and energy aspect.

Otherwise, recent Intel based Mac (e.g. Mac mini) is pretty fast, quiet and low energy consumption. They can run OS 9.0.4 with SheepShaver at pretty fast speed.