is PCI 2.1x16 cards compatible with x16 slot?

[FPSFanboy]

i am going to upgrade my GPU.
my current GPU is a GeForce 8400 GS
i'm looking forward to the HD 66770

 

[manofchalk]

All the PCI-e revisions are forward and backward compatible, your good to go there. The 16x refers to the number of physical connectors on the card and slot, if your card has equal or less than your good to go.