I mean is there any real advantage over PCI?
Absolutely ... YES !!!
PCI speeds can't really go any faster without the hardware costs rocketing, and even then no great improvement will happen.
The problem with parallel interfaces is you have to keep a bunch of signals in sync else the data gets read incorrectly.
All very prone to capacitance and inductance problems, but never mind the reasoning.
PCIExpress is a *serial* interface, so only *two* active wires are needed, so it's way easier to go faster .. easier, yup .. easier.
Also, PCIExpress *lanes* don't share bandwith, whereas *all* PCI devices must.
The end result being extra speed which also applies to externally connected devices, not just internal. This includes AGP cards too, they will fall by the wayside.
PCIExpress will become the next standard.