It is very likely that Republicans will pick up no more than one seat, while Democrats may pick up as many as seven seats--and very likely, counting losses, somewhere between one and five. That will give them (counting Bernie Sanders and the apostate Joe Lieberman) at the very worst their existing 51-49 margin, but more likely somewhere between 53-47 and 59-41. That is not enough to withstand a filibuster on controversial labor law reform legislation, but probably enough--with a Democrat in the White House--to pass some version of national health insurance.
I'll note one thing here. The political instincts of elite decision-makers are based on experience, and experience suggests that the 2000-2004 map is pretty fixed. Red states are red states, and blue states are blue states, swing districts are swing districts, etc. That's just natural. Humans just like to think that if something happened yesterday and the day before yesterday and the day before that, it's going to happen again tomorrow. It gives people a feeling of control.
The problem of course, is that making political assumptions this way precludes the possibility that large numbers of people have changed their minds. And yet, that's exactly what's happened. Here's a map by Professor James Stimson on public policy preferences from 1952-2004.
Democrats are going to pick up lots of Senate seats, which is a symptom of a dramatic change in public sentiment. Elite decision-makers haven't picked up on this yet, so some of them are going to be removed from office.