From the 3.5 SRD
Favored Enemy (Ex): At 1st level, a ranger may select a type of creature from among those given on Table: Ranger Favored Enemies. The ranger gains a +2 bonus on Bluff, Listen, Sense Motive, Spot, and Survival checks when using these skills against creatures of this type. Likewise, he gets a +2 bonus on weapon damage rolls against such creatures.
At 5th level and every five levels thereafter (10th, 15th, and 20th level), the ranger may select an additional favored enemy from those given on the table. In addition, at each such interval, the bonus against any one favored enemy (including the one just selected, if so desired) increases by 2.
If the ranger chooses humanoids or outsiders as a favored enemy, he must also choose an associated subtype, as indicated on the table. If a specific creature falls into more than one category of favored enemy, the ranger’s bonuses do not stack; he simply uses whichever bonus is higher.
For 3rd edition, if I remember correctly, it is the reverse of what you said, your older ones get stronger when you pick up a new one.
Yet one more reason I prefer 3.5.