Recent performance tests I have read suggest a rough equivalence between the new Core 2 Duo GHz rating and that of an PPC G5 chip. The Core 2 Duo is generally stated by Intel to be about 40% faster per GHz than the Pentium that you likely have in your Intel PC, and so the logic suggests that your 1.4 GHz Mac is roughly equivalent to a (1.4 GHz + 0.40 * 1.4 GHz) = 1.96 GHz Pentium, So, your Pentium PC is still the faster part in more or less equivalent GHz, but it is a lot faster than the 1.4 GHz number suggests when you stack that up against a 3.0 GHz number for the Pentium. However, that gets us part way to the performance difference you are seeing.
Next, as has been pointed out above, Mac OS X is simply more efficient, and less bogged down with anti spyware stuff, so there is less drag on the response of the system from the OS itself. That leaves more CPU for the applications themselves to take advantage of.
Finally, and this likely key, programs like iTunes in particular have been extensively optimized for the PPC chips. I really noticed this when I got my Mac. iTunes on my 2.3 GHz PowerMac just flies relative to iTunes on my older 3.0 GHz Pentium PC. Using my "40% faster" number from above, my 2.3 GHz PowerMac is roughly equivalent to a 3.2 GHz Pentium, so it is a little faster in equivalent GHz, but that is not nearly enough to explain the radical difference in speed. That is wholly the result of optimization in iTunes.
By the way, the same is true of hefty monsters like Photoshop, where many filters run nearly 2x as fast on my 2.3 GHz PowerMac as they did on my 3.0 GHz Pentium.
Apple, and many of the software vendors out there, have done a WONDERFUL job in letting the PPC chips reach their full potential.