Yup sure is! That's what's nice with ATI's dual card config. You dont need to have the same card. As long as one's a Crossfire card and the other is "Crossfire ready" and the X1800 are corssfire ready.
...pissed off a Marine huh? great! I'm a soldier so...HOOAH!
but forreals... don't mess with any of the armed forces. we stick together when threatened. Muahahaha!
urm, he's asking wether he can use a X1900 with a X1800, and i dont know the answer to that but i know ATi can disable pipes to match two chips to get crossfire workin, so maybe, but performance wont be great - more like performance of a single x1900? cause of 2xX1800's? i dunno LOL
Actually, the Crossfire cards do not downgrade. They operate at full speed, and performer internal config testing to determine how much of a load each card can handle, then instead of splitting the processing evenly, the beefier card gets more of the work.
So granted, matched is best, but unmatched does not downgrade the better card.