PPC applications run under Rosetta, including drivers and other programs or processes which have some effect (keeps getting to be smoother and less with each update, which are for all purposes, over now).
Power Mac G5
(June 2004) (PowerPC G5 (970) @ 2.0GHz x 2) 155.0
(June 2004) (PowerPC G5 (970) @ 2.3GHz x 2) 170.8
(June 2004) (PowerPC G5 (970) @ 2.5GHz x 2) 179.9
(June 2004) (PowerPC G5 (970) @ 2.7GHz x 2) 189.4
(Late 2005) (PowerPC G5 (970) @ 2.5GHz x 4) 279.0
Mac Pro
(Intel Xeon @ 2.00GHz x 4) 263.8
(Intel Xeon @ 2.66GHz x 4) 330.3
(Intel Xeon @ 3.00GHz x 4) 372.3
Comparing Mac models - Dec 2006 - Geek Patrol
I think that sort of says what differences there are. More expensive ECC memory being another. Only the G5 Quad came close.
Mac Pro 2GHz 4GB 10K Raptor 23" Cinema Mac OS X (10.4.9) WD RE RAID Aaxeon FW800 PCIe MDD-G4 APC RS1500 Vista