CrossFire can be implemented with varying-GPU cards of the same generation (this is in contrast to Nvidia's SLI, which generally only works if all cards have the same GPU). This allows buyers who have varying budgets over time to purchase different cards and still get the benefits of increased performance. With the latest generation cards, they will only crossfire with other cards in their sub series. For example, GPU in the same series can be crossfired with each other. So a 5800 series GPU (e.g. a 5830) can run together with another 5800 series GPU (e.g. 5870). However GPU's not in the same hundred series cannot be crossfired successfully. (e.g. a 5770 cannot run with a 5870)
