"I work at a university, and our dual 2Ghz G5 just arrived. I just did a quick iTunes benchmark, converting "Moment's Notice" (Coltrane) from AIFF, on the hard drive:

I just watched for the highest reached conversion speed factor:

Dual 2Ghz G5:
converting to 160 MP3: 45.6x
converting to 160 AAC: 33.5x

My 12" PowerBook G4 867Mhz:
converting to 160 MP3: 11.5x
converting to 160 AAC: 8.8x

The G5 is standard factory configuration.
The PowerBook is running Panther 7B53.