Yes, I would suggest the two 5770s. You can easily get a good 5770 at $150-160 now, and the X-Fire should be able to power what you want.
What's your resolution? I assume you're creating this system for gaming?
Also, the scaling of crossfire has really improved from 4000s to 5000s.
Make sure that your system has an adequate motherboard and a good, solid power though (for either X-Fire or the GTX470).