If you decide to kill Berengar, you gain 4500 XP immediately after the dialogue finishes rather than once you manage to actually kill him. If you decide to spare Berengar, you gain 6500 XP immediately after the dialogue finishes and Berengar's inventory with the exception of his medallion.

Is there any way to save Berengar?

SAving Berengar is possible, althought, like KiNgBrAdLeY7 said - doesn't give anything. Players usually just treat it like a challange or achievement to save him. :) Mostly...

Is Berengar dead in Witcher 1?

Unfortunately, he perished, but his sacrifice was not in vain. There is no additional entry to Berengar's character section if the player prevents Javed from killing him.