No. They have to be the same GPU type. I'm not sure whether they must have identical VRAM quantities, or whether a mismatched pair would operate at the RAM level of the lesser card. FAQ
"Can I mix and match graphics cards that have different GPUs?
No. For example, an XXXGT cannot be paired with a XXXGTX in an SLI configuration."
Some people use a second nVidia graphcs card as a dedicated PhysX processor, but I've never tried it.