You can only crossfire cards of the same model. EG: two 5770's. You're going to have to use a dummy plug.
Actually it is slightly less restrictive. You should be able to crossfire cards with the same hundreds number, e.g 5830 and 5850 can be crossfired.
This^
You absolutely cannot CF 2 different generations (Ex 6xxx with 5xxx), and you cannot CF 5800 with 5700. However, you can CF 5800s with 5970!