Amdahl's Law is one of the fundamental principles of computer science and basically states that efficient systems must provide a balance between CPU performance, memory bandwidth, and I/O performance. At odds with this, is Moore's Law which has accurately predicted that semiconductors double their performance roughly every 18 months. Since I/O interconnects are governed by mechanical and electrical limitations more severe than the scaling capabilities of semiconductors, these two laws lead to an eventual imbalance and limit system performance.
This would suggest that I/O interconnects need to radically change every few years in order to maintain system performance. In fact, there is another practical law which prevents I/O interconnects from changing frequently - if it am not broke don't fix it.
Bus architectures have a tremendous amount of inertia because they dictate the bus interface architecture of semiconductor devices. For this reason, successful bus architectures typically enjoy a dominant position for ten years or more. The PCI bus was introduced to the standard PC architecture in the early 90's and has maintained its dominance with only one major upgrade during that period: from 32 bit/33 MHz to 64bit/66Mhz.
Personal Computers or PCs are not pushing the bandwidth capabilities of PCI 64/66. PCI slots offer a great way for home or business users to purchase networking, video decode, advanced sounds, or other cards and upgrade the capabilities of their PC. On the other hand, servers today often include clustering, networking (Gigabit Ethernet) and storage (Fibre Channel) cards in a single system and these push the 1GB bandwidth limit of PCI-X.
