There is some info here:
http://en.wikipedia.org/wiki/Pci_express
The x1 slot is intended to be a replacement for a PCI
slot. It has a bandwidth of 250MB/sec in each direction,
and the interface is bidirectional.
The x16 slot is intended as a replacement for the AGP
slot. It has a bandwidth of 4000MB/sec in each
direction, and the interface is bidirectional.
One difference about x16 PCI Express, is there is no
GART as in AGP. The x16 PCI Express slot can be
used for video cards, but it can also be used for
RAID controllers or 10GbE network interfaces. So the
function of the PCI Express slots is not fixed. (For
example, there are even video cards from Matrox
that plug into a PCI Express x1 slot.)
Just recently, a format for PCI Express cabling has been
approved, so now it is possible to extend the lanes
via cables. So someday, you may be seeing PCI Express
used to extend I/O bandwidth, outside the computer.
"PCI Sig approves external cabling spec 1.0"
http://www.theinquirer.net/default.aspx?article=37528
Paul