The connection that bridges your video card to the motherboard on your computer is known as the PCI Express bus.
With the introduction of PCIe the computer world saw massive improvements in data rate,error reportings,less overhead compared to previous standards like the AGP
The PCI Express standard is developed and maintained by a group of 900 companies also known as the PCI Special Interest Group .
Their most recent standard is the 3.0 gen bus,or PCI Express Gen 3.0. The first devices to use it are the video cards from the Southern Islands family of GPU’s,developed by AMD.
The PCIe standard is categorized in connections with various lane numbers or speed. We have slots with 1x,4x or 16x lanes.
The PCIe 16x is dedicated to video cards for now,with other devices using the 1x and 4x lane connections .
Theoretically the PCI Express X16 bus is about 200% faster than the old AGP 8X bus thanks to a higher bandwith and less overhead, tough it is hard now to give a comparison since the AGP slot has been completely phased out.
It is hard to make a practical comparison between the two connections since the AGP standard is no longer produced and the connection is most burdened when the video card is limited on memory by the application,or other graphical features.
But comparing PCIe generations between themselves is much harder than PCIe versus AGP.
With each gen,the PCIe connection saw a doubling in data rate and bandwith,but consumer needs remained more or less the same,applications that required such high bandwith were rarely introduced.
If you are sporting a single card,it is very unlikely that you can saturate the bandwidth of a PCIe 2.0 bus,never mind a 3.0 one.
The only users who benefit from the larger bandwith are multiple card users,with rigs that use SLI or crossfire and who generaly play at much higher resolutions,more display outputs.
PCIe gen 3.0 is also recommended for developers with heavy compute needs,like AMP and c++ developers.