It is possible to Crossfire two cards that aren't exactly the same but the problem is the getter card will drop down to the level of the least performing in speed AND memory.
So buying a more expensive card is a waste. Plus I'm not entirely sure there would be no issues.
Why are people with great computers upgrading so often? I upgrade every three years and just play the games that can run on my system and love it.
Don't forget you have to have a kick-ass CPU to keep up with a good Crossfire or SLI. Not to mention the noise and heat...