I suppose the question then becomes: What can they do? Before I get flamed, let me make a disclaimer: I am not trying to play the expectations game here, so don't even start with that. I'm interested in what's actually possible and likely.
From what I've read, they plan to propose $100B in cuts right off the top. They may also have weekly spending cut proposals. A discussion on an immediate return to 2008 spending levels has been discussed as well. By the time Congress comes into session, I expect that at least most of the Bush tax cuts will have been renewed--but if not, they'll push that as well.
The big pieces...repealing Obamacare, major tax reform, social security reform, etc...I don't see how they get done even if they have both houses. They will lack a veto proof majority either way. I suppose the caveat would be how upset the Blue Dog Dems are with Obama and the party. If they peel them off as well, they may end up having a big enough majority to overcome any veto by Obama. I don't think it's likely, however.
In the end, I see them being to slow down things (spending, quasi-nationalization) through controlling the purse strings and committee chairmanships. They still won't be able to do much about the "Czar Republic" that governs by regulatory fiat through the Obama Administration.
Unfortunately, I think that both Obama and the GOP are going to try to get into "who's got a bigger set" contest by blaming each other for obstruction. Obama will try to run on Republican legislative obstruction in 2012. The GOP will try to run on Obama obstructing the will of the people as expressed by Congress. Who wins is anyone's guess, but it's going to be ugly. My hope is that enough will happen (or heck...NOT happen) to get the economy back on track and get the debt situation under control.