No, you made the right decision. Even though the 4850x2 (HD5850) is stronger than the HD4890, you would:
Have no room for upgradability
Use more power
Output more heat
Deal with a lower minimum FPS (Not really bad tho..)
Now, however, you can buy another HD4890, and crossfire those. A pair of 4890's should beat an HD5870, I think.