In a "realistic" game, I don't think a serrated blade makes enough of a difference to bump up damage or crits any. The difference between a serrated longsword and a regular one, for instance, is a lot less than the difference between a longsword and a short sword. In game terms, the difference between a short sword and a long sword is about 1 damage per hit (and some auxiliary effects of the S to M weapon size).
In a "spikey helmets are good" game, I can see serrated swords being exotic weapons that have +1 crit range OR a promotion to the next higher damage die (the latter being a more potent advantage, as any greatsword wielder could tell you). Increasing crit multiplier should be reserved for more drastic processes, like hollowing out the blade and partially filling it with mercury. Or turning the weapon into an axe. Then you'll need to consider madness such as serrated mercurial greatswords (of course, the DM could rule that all mercurial greatswords are already serrated, so the standard stats apply)!
Even in the second case, I would come up with some reason why everyone isn't toting around serrated blades (if everyone IS, then the serrated ones are the NORMAL weapons and should keep the old statistics... and straight blades would get the "coolness bonus")! Maybe the serrations get caught in the scabbard a lot, and it requires a standard action to draw one (or a move action if you have Quick Draw). As a related example, mercurial swords are simply another name for "githyanki silver swords" in my campaign, so it takes a lot of guts to adopt one for normal use. Given that mercury is also called "quicksilver", relating this substance to a race known to carry "silver swords" is not difficult.