Well if you want to go xfire, just make sure your PSU can handle both, that you have the plugs needed. I haven't looked it up off the top of my head, but some of those cards take an 8-pin, and some take 2x 6-pins, etc. Make sure you have the physical connectors to power them or can get adapters.
2x cards will also generate a bit more heat, too. They can also be buggy in some games that don't support xfire as much.
Which brings me to the next: Does AH support xfire? If not you won't gain a benefit from 2 cards like that. It would run like a single card. Might check with skuzzy on it, since the search function is disabled until they fix it.