Cross fire is only able to used with chips that have the same model type, (7970 or R9280 or R9290, even the X series). You can even do this with different brands of card. The slowest card is the default, and the faster card will not help the slower card.
Use the fastest card, or the best card for you monitors, in the number one PCI-E slot on your Mother Board.
Only use the ports on the first card, as the second card is only an add on to the first card, via the bus.
