Maybe not simple, but...

SRD says that "If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly."

If I'm not mistaken, a wizard with 3, 4 or 5 CON has a -4 or -3 Constitution modifier and gets 1HP per level (since it's a minimum).

Q636: If a wizard with 3 CON manages to reach level 8 and increases is CON to 4. Do his HP go from 8 to 16? And to 24 if he finds a +2 amulet of health in addition?