As my copy of savage species is absent, I'll be using the variant race rules from Unearthed Arcana and some inspiration from Races of Destiny.

The first task is to find an urbanized elf. While they don't exist, the various guidelines from the terrain/environmental races is useful, and basically says "exchange abilities for equal value."

The Elf is:

+2 Dexterity, -2 Constitution.

Medium: As Medium creatures, elves have no special bonuses or penalties due to their size.

Elf base land speed is 30 feet.

Immunity to magic sleep effects, and a +2 racial saving throw bonus against enchantment spells or effects.

Low-Light Vision: An elf can see twice as far as a human in starlight, moonlight, torchlight, and similar conditions of poor illumination. She retains the ability to distinguish color and detail under these conditions.

Weapon Proficiency: Elves receive the Martial Weapon Proficiency feats for the longsword, rapier, longbow (including composite longbow), and shortbow (including composite shortbow) as bonus feats.

+2 racial bonus on Listen, Search, and Spot checks. An elf who merely passes within 5 feet of a secret or concealed door is entitled to a Search check to notice it as if she were actively looking for it.

Automatic Languages: Common and Elven. Bonus Languages: Draconic, Gnoll, Gnome, Goblin, Orc, and Sylvan.

Favored Class: Wizard. A multiclass elf’s wizard class does not count when determining whether she takes an experience point penalty for multiclassing.

A con penalty doesn't seem appropriate for a "poison resistant" race. So we'll swap out con for -2 strength.

Base speed is fine.

We'll trade immunity to magic sleep effects and +2 racial against enhancment for:

"Immunity to nonmagical poison and poison effects, and a +2 racial saving throw bonus against magic poison, and all kinds of disease."

Drop the weapon proficiency line, due to the scope of the posion versus sleep. (Force the player to take a class/burn a feat for that like everyone else)

And I think it's quite fair as a race. Useful for those who want to use poison, but not horribly broken unless designed that way, and certainly no more broken than most other elves.

Poison Elf:

+2 Dexterity, -2 Strength.

Medium: As Medium creatures, elves have no special bonuses or penalties due to their size.

Elf base land speed is 30 feet.

Immunity to nonmagical poison and poison effects, and a +2 racial saving throw bonus against magic poison, and all kinds of disease.

Low-Light Vision: An elf can see twice as far as a human in starlight, moonlight, torchlight, and similar conditions of poor illumination. She retains the ability to distinguish color and detail under these conditions.

+2 racial bonus on Listen, Search, and Spot checks. A poison elf who merely passes within 5 feet of a secret or concealed door is entitled to a Search check to notice it as if she were actively looking for it.

Automatic Languages: Common and Elven. Bonus Languages: Draconic, Gnoll, Gnome, Goblin, Orc, and Sylvan.

Favored Class: Any. When determining whether a multiclass posion elf takes an experience point penalty, his or her highest-level class does not count. The huge diversity of experience in cities have broadened the Poison Elves traditions relative to their more focused forest cousins.

Depends on what improves the fun of the table

For a fairly trivial example, contact other plane plus some really basic statistics should be capable of knowing anything with high degrees of accuracy. Someone with 30+ Intelligence and maxed ranks in Spellcraft should probably not have any trouble figuring it out.

But not only is this unfun, if the player knows everything, it’s almost impossible for the DM to handle. He has to come up with answers to all these questions, even things that he never planned on fleshing out (or, at least, expected prior warning if it were likely to become necessary to flesh out).

So there are limits on how smart your character can be before it starts getting in the way of the table. Where that limit is, however, depends largely on the tables itself. I actually know people who actually have characters that do statistical analysis of contact other plane responses, and their DM is ready for that. I think that’s crazy, but they enjoy the game. I wouldn’t allow it at my table, nor would I enjoy it if I were another player in a game where that kind of thing was going on.

But I think it is impossible to generalize this across all tables.