PCI has gone through several changes over the years, including 64-bit versions at both 33 MHz and 66 MHz. PCI-X increased the 64-bit parallel bus to 133 MHz, and is backward-compatible with previous PCI versions. Meanwhile, Gigabit Ethernet, 2-GB Fibre Channel and Ultra 320SCSI have pushed the expansion-slot envelope.
These days, parallel architectures come up short in real estate and total bus bandwidth. PCI uses a shared bus, which limits the total bandwidth regardless of the number of slots. Server manufacturers have been required to install multiple buses to circumvent this problem.
PCI-E, however, offers ample bandwidth to meet the growing demand (for more on the PCI-E spec, see ID# 1509rd2). Taking a layered approach with packetized data transfers, it lets you use various physical mediums, such as copper or optical. Moreover, it's compatible with the current PCI specification.
The performance of a PCI-E device is characterized by the number of "lanes" it has. Each lane can carry throughput of 250 MB per second (excluding overhead) in either direction. PCI-E cards initially will come in various configurations based on number of lanes--in 1x, 4x, 8x and 16x configurations. A 16x configuration, for instance, will have a total throughput of 4 GB per second, excluding overhead.
Because PCI-E is a point-to-point serial connection, total bus bandwidth is no longer an issue. Each card is guaranteed its bandwidth, and you don't have to worry about oversubscribing as with PCI and PCI-X. Unfortunately, few PCI-E cards and slots are available so far.