The best case scenario would have Jackson pushing the disorganized Union forces off Cemetery Hill on the first day, before they had time to consolidate the position. Recent evidence suggests that even if Jackson had been there, he could not have organized an attack, mainly because the Confederates created a traffic jam when they flooded into the town that made it impossible to redeploy.
A win at Gettysburg wouldn't have won the war. Grant took Vicksburg and cut the Confederacy in two almost on the same day. He would have still come east and employed the same tactics that eventually won the war. The Army of Northern Virginia never had enough manpower to completely destroy the Army of the Potomac, or even cut their numbers down appreciably without suffering similar losses. Even if the AoP had been cut down to half its size, it could still have fallen back on the Washington defenses, and the Union could replace its losses, while the ANV could not.
The whole issue of what would have happened if the South had won the war has been done to death. Harry Turtledove (he's the guy that wrote the AK 47 story someone else mentioned, but that novel is not part of the series) explored the idea in a series that chronicled not only a second civil war, but both world wars and (I believe) the 50's and 60's as well.
