My sense of things is that, barring another surprise, that the Republicans will just hang on to the Senate, (although I think George Allen is in trouble in Virginia). I also believe (but with less certainty) that the Republicans will probably squeak-by in the House of Representatives as well.
Blogging Caesar and Real Clear Politics (see sidebar), have details, or plenty of links to places to find detailed polling information, if you're interested.
Anger is doing a lot right now for the Democrats, as is the eagerness of the national press to be helpful. Losing by just a little would not be all bad for the Republicans: the Democrats completely lack a program, other than Bush hatred, and taking over the Congress would put them in the completely unenviable position of acquiring political responsibility without the actual ability to accomplish anything. If Democratic policy preferences could survive filibusters and other parlimentary delaying tactics, they would still face the Bush veto pen.
The President, for his part, would be able to take a leaf from Bill Clinton's book on stymying Newt Gingrich and his Republican majorities. He might recover some of the public esteem he has forfeited since 2004, and be in a position to blame the Democrats for problems in 2008.
Finally, I don't see how the Democrats can avoid trying impeachment, if they get a majority or even something close to it. Their base and contributors will demand it. But they don't have the votes in the Senate to do it -- not without significant Republican defections, which are unlikely. I think they'd wreck themselves trying it.
So, there is a possible upside to doing badly. . .but for all that I pray it does not happen. I think it would mean a virtual collapse of the war effort, mostly because of the encouragement any gains by the Left would give to our enemies in Iraq and in Afghanistan. Conversely, the Republicans avoiding disaster now would discourage both the Left here, and our foreign enemies, and offer the government an opportunity to deal from a position of strength.
We need a strong government just now, not paralysis. Here's hoping. . .