The best conceivable outcome I can see for Democrats is them holding all their seats except Nebraska, while picking up MA, IN, AZ and NV (plus have King caucus with them). That would mean a net gain of 4 seats and a 57/43 Senate.
Of course, that would mean every close race breaking in favor of Democrats. That seems very unlikely to happen.
I think the best realistic result we can hope for is 55/45. This should be enough to survive in 2014, barring a 2010-like wave.
2014 should be another Republican wave, but 2016 should be a Democratic wave with the Senators who were election 2010 coming up for re-election. Of course, 2014 and 2016 all depend on how the next Administration is doing. If Obama wins this year and is doing poorly, 2014 and 2016 could be disaster for the Democrats. If Romney wins and is doing poorly, 2014 and 2016 could be disaster for the Republicans. Right now, I see Obama winning in 2012 and losing seats in the House and possibly the Senate in 2014 and 2016 being a wash with the Republicans gaining control of the White House, but with significant Democratic gains in the House and the Senate. The increase in the House may be enough to put Steny Hoyer or Nancy Pelosi back in the Speakers chair come January 2017. I see the Republicans holding the House at least for Obama's entire second term.