It depends on what series you are looking at.
GTAIV -> GTAV = much better graphics, improved performance and going from a sub-HD res to 720p on the PS3.
Forza 2 -> Forza 4 = Same res, much better graphics.
Gears of War -> Gears of War 3 = Same res, better performance, much better graphics
Halo 3 -> Halo 4 = Sub-HD to 720p, better AA solution, improved graphics.
Uncharted -> Uncharted 3 = Same res, better AA, better performance, much better graphics.
PGR3 -> PGR4 = Sub-HD to 720p, much better graphics.
Tomb Raider Underworld -> Tomb Raider 2013 = Sub-HD to 720p, much better graphics.
Mass Effect -> Mass Effect 3 = Same res, much better performance and graphics
Splinter Cell: Conviction -> Splinter Cell: Blacklist = Sub-HD to 720p, better graphics
FIFA 2006 -> FIFA 2014 = Sub-HD to 720p, much better graphics
Oblivion -> Skyrim = Sub-HD to 720p, much better graphics.