For Nvidia, you have to have the same card. (e.x. 550 + 550)
For AMD/ATI, you can have a different card in a generation. (e.x. 6870 + 6850)
That isn't necessarily true. Many cards from Nvidia with the same GPU configuration or at least a functionally identical GPU configuration can do SLI, although some take a little extra work. For example, various G92-based cards can do SLI and there's also a way to get GF104 and GF114 cards with the same core configuration to do SLI.
Furthermore, AMD's CF is not on a per-generation thing. It's generally on a per-GPU basis (differing from Nvidia's case in that it can be the same GPU, but usually doesn't need to be the same configuration). Yes, the 6850 and 6870 can do CF, but they're all from the Radeon 6xxx generation and they can't do CF with most other cards of that generation. Furthermore, like Nvidia, there are a few examples where cards of the same GPU but different generations can do CF (5750/6750 and 5770/6770).
To the best of my knowledge, the GTX 560 Ti 448 core can only do SLI with another GTX 560 Ti 448 core, but that's not the case with all Nvidia cards.