To a certain extent, that's true. The Mac Edition of the X800 is just the X800 flashed with a more Mac-compatible firmware and better Mac drivers (as far as I know).
But just about everything else is totally off the shelf now, especially in the Power Macs. They use DDR and DDR2 RAM, standard laptop and desktop SATA hard drives, and have PCI and PCI-E slots (in the Powermacs). So far, I've put in two sticks of generic PC133 RAM, a cheap DVD burner, and two new PATA harddrives in my Mac, and it has yet to scream at me about it.