Simply put, PCI-E is an extension of older PCI. PCI uses parallel paths to send signals whereas PCI-E uses serial paths to send signals.
x1, x4, x8, x16 are simply aliases for the speed offered by each of them. Higher the number, greater is the speed.
All that refers to is the number of PCIe lanes required (although technically you could use 1 lane if you didn't mind spending a few hundred on a shiny GTX 580 only to make it constantly wait for data). Whether it is specified or not, as long as you have a full-length (16x) slot on your motherboard, you can put any PCIe card in it that is compatible with the standard that your motherboard uses - 1.0, 2.0 etc.
January 20, 2011 10:12:25 AM
GT 240 and GTX 580 are both the same slot, why isn't the GT 240 x16, or why doesn't it say x16.