It wouldn't necessarily be more powerful than two 7950s, but overall, it would, imo, be the better choice, since you wouldn't have to deal with any of the headaches/hassles that come with running 2+ cards.
Given AMD's release schedule, you probably won't see anything new within a year, and the 7950s are starting to show a bit of age regarding performance these days (with higher res, i.e. 1440p+). Only probable release in the next year would be Nvidia's Maxwell.
You should be able to get about $220 for your 7950, so to upgrade from it to a 290 for only $180 isn't that shabby of a deal (not counting Ebay fees, which you could forgo if you sold locally).