PCI-X isn't a Mac thing. Apple tends to jump on new hardware long before the PC world. But it makes its rounds. Biggest example is USB, basically it was non-existant until Apple standardized it, and companies started making products to work with it, and pretty soon the PC world is all over it. But it's not a Mac-only technology. By controlling the hardware market Apple is able to standardize things, and make every new computer ship with that hardware. Because of that, companies are willing to take advantage of it, because every person who buys a new computer will have it. It also allows Apple to make software that all Mac users can take advantage of. iMovie came out when they standardized FireWire, iTunes with CD-RWs, iDVD with the DVD-RW/CD-RW. It works nicely.
Anyway, standard PCI ports have a 32-bit bus running at 33mhz. PCI-X is a 64-bit bus running at 133mhz. But I don't know of much that actually uses the slots as of right now. But they don't need to really, it helps make your Mac last much longer. Over 2 years ago I bought this G4 of mine, it came with a DVD-RW/CD-RW (1st in the world, w00t!) and Gigabit ethernet. When I bought this, I could add a 4 port gigabit ethernet hub with gigabit ethernet cables for $999.99. 2 years later, gigabit ethernet is only slightly more expensive than 10/100, and my DVD-RW/CD-RW is far from out of date. That's why I love Apple. They push ahead, supporting new hardware long before the PC world. Wow, Dell has wireless networks, only years after I've been using AirPort, suckers!
G5s will have Serial ATA, FireWire 800, USB 2.0, AirPort Extreme, Bluetooth, and PCI-X to push it long into the future.
Attaching the leaked G5 specs.