I would go with the 4890, the 4850x2 will suffer from lack of support just like the 2x's 4850's, so some games will scale well while others will have the performance of a single 4850, so honestly unless you check the reviews and see what games it scales best in, and those are the majority of games you are going to play. While the 4890 will be the same performance all around (aside from performance differences from game engines).
edit: also the 4850x2 is sapphires' design so you have to deal with their tweaked drivers. That was a big reason why I personally had no interest in it.