I really wish people would have an idea what they were talking about before they posted garbage.
The Atom doesn't blow the G4 out of the water. The reason your mini9 seems so responsive in mac os is because it is running a solid state hard drive that has very fast random read times. Any decent quality SSD
will give off this effect.
A 1.6ghz Atom is not the same as a 1.6ghz anything else, and hyperthreading isn't the second coming of processor technologies. Here's the cold hard truth:
Clock for clock, the Atom is about half as fast as the Pentium M processor (Centrino/dothan, whatever you want to call it). So a 1.6ghz Atom is about half as fast as a 1.6ghz Pentium M. In most benchmarks, the Atom is about on par with the later G4s. The plus to the Atom is that it does it while only drawing 2W TDP.
So there you go, your Atom basically = a G4, unless you are talking about heat or power consumption.