All these polls are from Gallup's tracking poll, all done within the same time frame.
http://edition.cnn.com/2000/ALLPOLITICS/stories/09/28/tracking.poll/">Gallup's Sept. 28th, 2000 tracking poll had both Bush and Gore tied at 46%.
http://www.gallup.com/poll/110674/Presidential-Debates-Rarely-GameChangers.aspx">Gallup's Sept. 28th, 2004 tracking poll had Bush leading Kerry
http://www.theleftcoaster.com/archives/002757.php">52-44, or an eight-point difference.
http://www.gallup.com/poll/107674/Gallup-Daily-Election-2008.aspx">Gallup's Sept. 28th, 2008 tracking poll has Obama up 50-42, or an eight-point difference.
So this is how it breaks down in margin:
Obama is doing eight points better than Gore was in 2000.
Obama is doing 16 points better than Kerry was in 2004.
There are some differences, however. The first, both Gore and Kerry's results were pre-debate. Kerry and Bush held their first debate on Sept. 30th, while Bush and Gore held their first on October 3rd.
Kerry gained ground after the first & second debates, tying Bush prior to the third. However, by the time the debates were over, Bush had regained the lead and held it all the way through. Gore, who was tied with Bush a few days before the debate, lost ground after the first, second and third debates and never really recovered.
So Obama is performing far better than Kerry & Gore were at this time. However, both races did tighten in the end, with Gore actually winning the popular vote and Kerry only losing the popular vote by 3 points nationally.