A 3.0 card will work fine in a 2.0 socket. A 2.0 card will work in a 3.0 socket. It just defaults to the lower speed. As for a card, either a gtx 760, or see if you could find a gtx770 on sale.
I prefer nvidia to AMD as they update thier drivers all the time and they generally run cooler
I think a GTX 770 would be your best choice. The AMD cards are horrible value right now because of the bitcoin inflation. You should double check about the duel monitor, but I'm pretty sure the GTX 770 will support it.