You can crossfire it with the other hd6450 but not with the 7850. To crossfire, they must both be the same series. That said, if you were to crossfire, say, a 7850 with a 7870, the 7870 would be downclocked to match the 7850, so for best gains, it's better to match the same model cards. As for the biggest performance, the single hd7850 would be the one. Also, check to see your psu is powerful enough to run the crossfire set up.