When it comes to congress that kind of poll isn't too telling. Sure, it means the republicans have picked up steam, but
here is another one also done by Gallup that shows the favorable ratings of both parties at near all time lows. The democrats are sitting at 43% while the republicans are at 36%.
So I think that previous poll in the generic election shows a couple of things: First, it shows that republicans are more excited by the upcoming election then democrats. Second, it shows that generic is sometimes better than naming a specific candidate.
Take for example the 2012 presidential election polls. When they do a poll that put Obama Vs Generic Republican it is actually a pretty close poll. However, when they do the poll where they match him against the big named front runners like Romney, Palin, Huckabee and Gingrich he beats them all by a decent margin.
Congressional elections are such local affairs that there could be a handful of states that are very excited and will could be very pro-republican while the many others are just not so the polls will show overall the republicans are way out in front when that might not be the case.
All that said, I think the republicans with either take back the house by a very small margin or they will come within a few seats of doing so.