hi, wats the difference between a Sapphire Radeon X800 GTO² 256MB DDR3 PCI-Express and a Sapphire Radeon X800 GTO, wats with the number 2 above the O?

also when purchasing a motherboard what is the difference between a

PCI Express x16 , PCI Express x1 and a PCI Express x4?

which expansion port would this card go in?