I don't expect Nader to do too well.
Obama has a lot of bases covered that Gore and Kerry did not in 2000 and 2004. Nader essentially is running as a protest vote but since there is a halfway decent candidate for the Democrats, I'm not sure why he is running. If it was Clinton, I probably would vote for Nader. He might get 0.5 to 1 percent of the popular vote.
Barr will do worse. Lolbertarians will talk about voting but will never get to the polls. I don't see them fawning over Bob Barr like they are Ron Paul so maybe he will get 0.5 percent or lower.
I also think that Gore did lose in 2000. If you could narrowly beat Bush by such a small margin, you don't deserve to run the country anyway.
Obama - 51%
McCain - 47%
Barr, Nader, Socialists, Nazis, etc. - 2%