PCI compatibility... HELP!

can i use a  Powercolor Radeon 7850(PCI 3.0) GPU on an MSI- 970A- G46 motherboard (PCI 2.0 slot) ? HELP

Yes

Yes, Pcie 3.0 cards are backwards compatable with Pcie 2.0 slots.

so are 2.0 cards in 3.0 slots.

the only difference is the controller on either end.

and the controller on 3.0 allows for 2x the bandwidth over previous 2.0

EDIT: Although it seems like 3.0 is needed for more FPS. games and cards these days do not utilize the full bandwidth of Pcie3.0. making it almost pointless for current GPUS, But deeply needed for Gigabit network controllers

That comes to 16GB/s on pci-e 2.0 and  32 GB/s on pci-e 3.0

hmmm no, pcie 3.0 is 8 gigatransfers per lane, my point being it's a little more complicated than that