As the first PCI Express 3.0-compatible motherboards slowly begin to seep into the marketplace, the next major revision of the interconnect is already in the works. PCI-SIG, which is responsible for developing the PCI Express spec, has announced some preliminary details about PCIe 4.0.
For starters, PCI-SIG says it has decided on a transfer rate of 16 GT/s for the next-generation interconnect. A feasability study purportedly showed that 16 GT/s can be achieved over copper wires, at roughly the same power levels as PCIe 3.0, using chips fabbed with "mainstream silicon process technology." The speed increase won't compromise backward-compatibility with previous iterations of the PCIe spec, either, so today's PCIe devices should work just fine in PCIe 4.0 slots (and vice versa).
PCI-SIG doesn't mention which encoding scheme the upcoming standard will use. Assuming the same 128b/130b system as with PCIe 3.0, a 16 GT/s peak transfer rate would translate into per-lane, per-direction bandwidth of just under 2GB/s—a two-fold increase over the PCIe 3.0 standard, and nearly four times as fast as PCIe 2.0. You'd be looking at about 31.5GB/s per direction for a PCIe 4.0 x16 slot.
While the transfer rate increase is the biggest news, PCI-SIG's priorities also include reducing power consumption and keeping costs in check. The group says it plans to "investigate advancements in active and idle power optimizations" with PCIe 4.0. In addition, PCI-SIG expects the upcoming spec to "address the many applications pushing for increased bandwidth at a low cost"—including, it says, desktops, notebooks, and tablets. PCIe 4.0 looks poised to become just as much of a mainstream interconnect as current iterations of the standard.
The bad news is that you'll have to wait a few more years for PCIe 4.0-enhanced devices. PCI-SIG doesn't plan to have the specification completed before "the 2014-2015 timeframe," and it may take at least another year until the first compliant products hit stores. For reference, the PCIe 3.0 base spec was made public just over a year ago. Although some of today's motherboards are compatible with it, I'm not aware of any commercial devices that actually take advantage of PCIe 3.0's higher bandwidth ceiling right now.