For gpu compatibility, you usually check for:
1. Whether your current psu meets the requirements of the card. If not, you would have to upgrade your psu.
2. The length of the particular variant of the card you're getting and whether your case has the necessary gpu clearance for it.
3. Whether your current cpu is powerfull enough to get the most out of your card.
I doubt there is a difference of the gpu performance from using pcie 2.0 instead of 3.0, so I would not upgrade the mobo just for that. The only reason I would upgrade the mobo is if I'm thinking of a cpu upgrade of a different socket.