I'm not sure why you would want to get a 5770 and a 5670 because you wouldn't be able tp run them in CFX. But either way I woild get the 5830 so you could possible upgrade to CFX in the future. Or if your motherboard supports it get a GTX 460 768mb. They are the same price as a 5830. Enjoy
I own 3 5770s in two different systems, one in crossfire. I have heard to avoid the 5830 for performance vs price reasons. I would recommend the 5770 or the 5850. The 5850 is a better buy for the performance, it is reported to overclock well. As beranimus suggested the GTX 460 is a great candidate, and scales well in SLI if your MB supports it.