Yes, PCI-E 3.0 is backwards compatible with PCI-E 2.0. Assuming the new RX480 does not introduce a new variant of PCI that is not backwards compatible; which is very unlikely, then the new lineup of Polaris GPUs should be compatible with 2.0 slots.
Yes. It will. There is very less difference between PCI-E 2.0 and PCI-E 3.0 which is negligible. In some cases PCI-E 2.0 was faster than PCI-E 3.0 . Therefore, you are good to go.
I currently have a
intel desktop board aa g10212
I72600k 3.4 GHz
8gb ddr3 corsair vengence
550w psu
And badly want a Rx 480
It will work just fine.
I recall a while back where games were tested on PCIe 1,2 and 3.
from what I remember the performance difference was very minimal.
Aha. Here's a link.
https://www.pugetsystems.com/labs/articles/Impact-of-PCI-E-Speed-on-Gaming-Performance-518/.