Novuake :
Raiderjoe :
Cards of the same series will CF; 79xx with a 79xx, 78xx with 78xx.
In nvidia's case it can only be done with the exact same card 660 with 660 etc.
Are you high?
No, he is not high, SLI only works with the same model card (e.g GTX 480 with GTX 480) Crossfire allows for two different cards of the same series (e.g 6950 and 6970) to work together.