3+3 = 6?

i'v heard a few people say that in the 6000 series of radeons that if you had a 2gb or 3 gb card running crossfire it was not double the gpu ram, but on the new 7000 series i hear they have fixed this problem and now you can have effectively 6gb of gpu ram can some one please go into a bit more detail of the problems that once was? or provide links explaining this?