Clock speed does not simply add up like that. In fact, back in the days when PPC was still being shipped, many lower clocked PPCs out performed higher clock speed Intels in benchmarks. Also, I am not sure the Pentium 4 would be outperformed by the G4 without seeing benchmarks, but perception is key. I find my 2006 iMac that I just bought to feel much faster than a HP Pavilion at school which benches much higher. May I ask, why would you be using ATA-66 on a MDD? To my knowledge, all MDDs shipped with an ATA-100 bus for the hard drive.