I would go with anything from the Gigabit upwards, including the QuickSilver. Avoid the AGP Graphics model as the choice of Graphics cards is limited due to the AGP slot not being powered.
I believe all of the G4's are good machines and all are suitable for a serious processor upgrade card.
just my 10c
P
I am looking for a used G4, I had a Quicksilver (Dual 1 GHz) which died and I need replace it inexpensively. I hope to keep this as cheap as possible (i.e. around 500), I know I will sacrifice performance, but such is life.
I heard that people have had more problems with the Quicksilver model dying than some others. Is the DigAudio or earlier models more robust?
Or is it worthwhile to risk another (lesser)Quicksilver?
I am not thinking of getting the mini because getting extra ram is too expensive and to use my studio display is an extra 100 bucks for an adapter.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic