Upgrading my graphics card

Hello

I have an old 2014 Area 51R2.  Currently running the original 2xGTX770's.  Up to recently this has been perfectly fine, but some new games and applications are starting to take its toll on the poor old things. 

I have read somewhere, because of the design of the case and card positions not all cards are compatible with this model.

Any advice on which of the latest cards I should be looking at would be much appreciated.  I think there are a few more legs in the old alien yet.

Not married to the the SLI config, i am told that most games do not design for this nowa days, so happy to go with 1 card.

much thanks

H