I would say probably a G4-400 or a G3-500 (unless you just mean the beat to death Photoshop tests using specific filters).

G3/G4 is more efficient then a P3 clock per clock, but its not 2x the speed either.

So basically I would think anything in a G4 tower could be reused (and a fast G3/G4 laptop), anything B&W or older should be recycled (asuming you mean for daily use and not for collecting purposes).