I also heard about the 4890 X2 but i dont think that its going to happen, seeing that in the time the 4890 x2 is out, AMD will have much better performing cards so a single 5890 could potentially be better than a 4890 x2. if it was gonna happen, it would have already.
The 4890 x2 will still likely beat the low in 5 series cards and if not then it will be the new low end budget card. I am pretty sure ATI already confirmed one was coming out and there is no reason not to release one at a low price point as it will be a 'high powered cheap alternative' to the 5XXX. Get two of those in quad fire and you have a rig that would likely beat any of the 5XXX until their x2 cards come out which likely won't be at first.
Anyways I would suggest you go with the CF 4890's. They likely are more powerful with the new drivers.