Just look at the stream processors. 5770=800, 5850=1440. This is more of an indication a speed than the other specs. If you're willing to go a little higher, the 5870 is double that of the 5770, basically two 5770's in one.
As you can see from my specs I own 2 5850s and 1 5770. The 5850 is a faster card but the 5770 is hardly a slouch. The 5850 requires 2 6 pin pci-e connectors while the 5770 uses 1. A lot depends on your other components. What is the maximum resolution of your monitor? I can play COD BO just about as well on the 1680x1050 20" screen with the 5770 as I can the 5850 on the 1920x1080 24" screen.