The links are good and the discussions in there bring up many good points about the difficulties of comparing a Mac to a P4 PC.
But, if I should guess, I would guess that an iMac G5@1.6GHz would be as speedy as a PC with a 2.5-3.0GHz P4 on applications that are properly optimized for both architectures.
On almost all games except maybe a few the iMac would probably be more similar to a 2.0-2.5GHz P4 with the same graphics card. The P4 has a triple advantage on games. The game code is always optimized for P4/AMD, Direct X development has gotten more resources than OpenGL development and OpenGL drivers for the different gpu's themselves has been optimized more for the P4/AMD/Windows environment.
Hopefully, game development for OpenGL, development of the OpenGL standard itself and optimization of OpenGL drivers for gpu's for Mac OS X will all see an increase when the Mac platform is more recognized as a profitable game market.