Difference between pci express x16 and pci express 2 1 x16

[spunnie]

i have a ATI Radeon HD 5450 HIS video card with a pci express 2.1 x16 interface. Will this card work in a pci express x16 bus add-in card connector?

 

[jaguarskx]

Most of the time, yes. It depends on the motherboard though. In those cases where a PCI-e 2.1 card does not work in a PCI-e 1.0/1.1 slot, it has been speculated that the PCI-e slot was not implemented properly by the manufacturer.

Basically, It's a shot in the dark with a small chance that it may not work.