i have a 5850 and did think about going xfire later, but i came to a possible conclusion that then i would have to sell 2 out of date gfx cards if i wanted something better, at the rate cards are progressing, i expect it makes more sense and saves money just to keep one really good card rather than 2 average cards
i have been thinking about this cause my gfx card is definitly the weakest link in my system, even tho its a new system, i will probably change my gfx card before getting an SSD for example