The move to PCI Express on the desktop begun with Intel's introduction of their 900 series chipsets back in June. The need for a high-speed serial point to point connections like PCI Express, stems from the fact that the aging PCI bus doesn't offer the performance necessary to sustain bandwidth hungry peripherals like HDTV tuner cards and high-end SCSI or SATA controllers. But until now, PCI Express wasn't available on AMD based systems.
So, why shift towards PCI Express when it hasn't shown any real-world performance benefits just yet? It's due to the fact that at 133MB/s the PCI bus has become one of the slowest links in a personal computer. Virtually every other connection in a system has been upgraded over the years, offering far more bandwidth than PCI. Not to mention, PCI is a shared bus architecture, so all peripherals on PCI share the available bandwidth. A 1GHz HyperTransport link, for example, offers up to 8GB/s of bandwidth, and the aging AGP spec has been upgraded to the point where it now offers up to 2.1GB/s of bandwidth. Even, VIA's own Ultra V-Link technology, which connects their Northbridge and Southbridge chips on a motherboard, offers almost 10x the bandwidth of PCI. The PCI bus, however, which is used for a myriad of expansion cards offers a maximum of a mere 133MB/s, and that 133MB/s is shared amongst all of the devices on the bus. Connect a fast SATA hard drive to a PCI RAID controller, along with a PCI connected Gigabit Ethernet controller, and an PCI HDTV tuner card, and saturating the bus is easy. Which is why dedicated PCI Express links have become increasingly more necessary.