The MSI R7970 is a "reference card", basically the same as any other branded reference card. Things you want to check is the specs of the 2nd card. If the clock speeds or memory are higher than your MSI the 2nd card will "down clock" to match you card.
Basically, any other 7970 will work.
What other brands are available at your local / online shops?
Adding the 2nd card is as simple as installing the 2nd card in the 2nd PCIx slot and using one of the Cross-fire cables to link the two cards. Then in Windows, open the AMD Catalyst center and enable Crossfire in the left-hand side menu.