Given how Bush just barely won in 2004, I'd say Gore loses. After 12 years of Democrats people would want change as they did in 1992. Gore and the Democrats would receive blame for the 2002-03 recession and I'm sure the GOP would still make gains in the 2002 midterms as a result. Gore would get a huge popularity bump from 9/11 and the war (and possibly getting Bin Laden), but again look to 1992 as a good reference guide. Bush's popularity was temporarily boosted by the war, but deteriorated by election day. Gore would be in a similar situation.
As for the Republican nominee, we basically get 2008 four years early. Giuliani begins as the frontrunner, but his disastrous strategy opens the door for McCain to earn the nomination. McCain would beat Gore, but whether or not he's re-elected depends upon his ability to handle Hurricane Katrina and the economic crisis.