Not really into benchmarking, but I just did:

Download lame

Downlaod AKsackl.waw

Placed both in ram:

":lame-3.99.5-morphos/lame :AKsack.wav x.mp3"

24s for the G5
31s for the PowerBook

If you were running at 600MHz you should get about 1 minute, 30s for 1800 (RAM speed is slow on those old G4s so you won't see anything near x3 by trippling clock).