Macs really aren't that good... graphics power compared to the PC is becoming lesser with the AMD64s 64-bitting and the HTs Dual Threading. With the PC Dual Processors, and both makers making 64-bit Processors the Intel version will surpass the G5 once more by at least 20% if not more. The AMD probably won't be far behind, if not ahead of the G5.
Its all down to compatibility - the PCs have a far larger user base and more peripherals, more software... its harder to integrate a new line of Processors into a User-End generally Mid-Range market like the Mac, becuase its user market is High-End generally rich looking for power - Mac Processor makers hence can go for more power and budget for it and make money still.
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.