That's fallacious. Just because it's onboard doesn't mean it doesn't use it's own chips, like a WinModem. PCI WinModems weren't onboard and yet they slowed your comp down.
And the Athlon XP3200+ is slower than the Intel P4 3.06GHz. The Dual 2GHz G5 is twice as fast as the Dual 3.06GHz Pentium4. The 64bit is obviously helping performance... and if it's not the 64bit then Apple REALLY knows how to design processors! Take your pick.