The 5850 is the second best card in ATI's newest generation. It performs about the same as the 4890, doing slightly better in some benchmarks. It does cost more, but it uses less power, runs cooler and quieter, and has support for DX11. You can read the Tom's Hardware review of the 5850
here.
You can Crossfire within the 5800 series, but it would be silly to since the 5870 will be downclocked to match the 5850s speeds. You'd want to get either two 5850s to save money, or two 5870s for the power.