Your quickest resources for this will be the Graphics Card Hierarchy Chart
and perhaps the benchmarks on the Graphics Cards Charts 2009
. The benchmarks aren't worth a ton without doing some research to understand why the results came out as they did, but they will give you at least some quick info.
Ultimately, if you want the performance of a 5850, get a 5850, get 2x4850 in CF. The 4890 and 5770 are going to be a step down in performance - perhaps not a very noticeable one, but a step down nonetheless.