REFERENCE:
http://www.ncpp.org/1936-2000.htm
For the popular vote:
Average error of major polls for US presidential elections post 1980.
2000 1.1%
1996 2.1%
1992 1.1%
1988 1.6%
1984 2.2%
1980 3.1%
As is typical of such things, averaging the results gives quite accurate results.
And for the coming election if you're interested;
Zogby Even
CBS Bush +2
Harris Kerry +1
Gallup Bush +4
Pew Even
IBD/TIPP Even
ICR Bush +3
NBC Bush +2
ABC Bush +1
Battleground Bush +2
Average forecast is Bush +1.3.
Now if the average margin of victory error is 3.8% (double the error per candidate) for the above 6 elections holds then the range of the average error would be Kerry +2.5% to Bush 5.1% giving Bush about a 67% probability of winning the popular vote.
But of course it's really about the electorals and that's damned close too.