well maybe not exactly but a 4890 performs similarly to a 5770 and 2 5770's in crossfire perform a little bit better that a 5850. its probabely less 1.9 but mine was a rough estimate.
Have a quick look at the plethora of reviews out there. A HD5850 whilst quicker than a HD4890 is no where near 1.9 times quicker.
HD5770 CF is also quite a bit quicker than a single HD5850 (when CF works properly).