Yes, you can have two different cards in one system. I have a 7870 GHZ Ed mining Litecoins slowly, while I game on a 7870 LE. Two different cores, so I can't crossrire, but both are in there running fine. Long as you have the room and the PSU you can do it, I had a 7970 in there too for a while, but my brother built another mining computer and needed it back.