Here's a performance chart with Apple vs Pentium and AMD. This one is not by Apple, so the scores are not biased. Just to show all that are pushed away by the low speeds, here's how they come out.
Even though the scores are not overwhelming, they do show that a Mac with half the speed (even though it's dual-CPU) can run better, or close to a P4 2.8GHz or a pair of AMD 1.6GHz MPs.
BTW. You might look and see that they are benchmarking 2 dual-CPU Macs, but if you look at the last few benchmarks, they runs apps w/o ANY multi-processing instructions at all.
They really should have put a G4 1.0GHz single CPU in there, just to compare. :P