Serial ATA & PCI-X
The specs of the hard drives are that they are 3.5 inches big (the same physical size as all desktop hard-drives), they spin at 7200rpm and they use the serial ATA interface. Performance of these drives should be near SCSI level but I suspect that it will not be too long before 10,000 rpm versions are made and sold. In the new PowerMac G5 cases there are two bays for the hard-drives compared to the four bays of my QuickSilver.
PCI-X appears to offer one 64 bit 133MHz slot and two 64 bit 100MHz slots. I think the PCI slots of my QuickSilver are 32 bit 33MHz with one at 66MHz. Considering that all G5 Powermacs come with 8X AGP for the graphics card, PCI-X may be overkill for anyone other than high-end users who might plug a high definition video card or a top-of-the-line SCSI card.