The short version: 250 – 4000 megabytes per second sustained bus throughput, 75 watts, backwards compatible with PCI 2.2. The variable speed is due to x1, x4, x8, x16 versions. 250MB per second per x. Current PCI is 133MB per second and 25 watts.
Although the bus is going to be backwards compatible with current PCI, the slots will not be. PCs will probably have both PCI and PCI Express slots for a while, although dual-use slots may also be on some boards. This will be the end of AGP, and if you have an AGP video card, you will need to buy a new PCI-E card.
Go here for much greater detail.
Tech people, please correct me if I’ve got any of this wrong.