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5770-Cooler,quieter and more future proof...
The 4890 is more powerful.
The 5770 is roughly equivalent to a 4870, but has advantages alredy pointed out:
* Significantly lower power consumption (which is also handy if you plan on adding a second later in crossfire)
* quieter
* The next gen of cards, supporting DX11, so will support nicer graphics in newer games (assuming it can run them!)
* Other minor features like supporting Eyefinity which you'll probably never use
If you're running really high resolution (like 1920x1200 or higher) *and* demand every ounce of power you can get, go for the 4890.
But otherwise, the 5770 is by far the more sensible choice.