Can anyone list what stats affect what in the 3rd edition rules (ie, what skills/abilities does intelligence control). Also to take levels in a secondary class what does your stat for it's primary req. have to be. I'm just wondering cause with IWD2 coming soon I was thinking over what my party would be.

Also anyone else think the clerics could be abused pretty bad. IE a tiefling rogue takes a level of cleric of mask for the blind fight feat and +1 to hide, etc. or a fighter using axes taking a level of cleric of Tempus?

That's a pretty big question ;) Ability scores in 3rd Edition affect just about everything in the game, unlike in 2nd Edition. Aside from the massive list of skills, here's a quick run down of the major ones:

Strength - Melee Damage and attack rolls, lifting/carrying capacity, any kind of difficulty check (DC) involving strength (such as a stuck door)

Dexterity - Range attack rolls, Armor Class, Reflex saving throw, combat initiative, and numerous skills (hide, move silently, open locks, tumble, etc)

Constitution - Bonus Hp per level (hit die), Fortitude saving throw and various skills

Intelligence - Skill points, conversation options, max spell level (wizard), bonus spell (wizard), spell strength (wizard) and lots of skills

Wisdom - Monk AC bonus (added to Dex bonus), strength of monk special abilities, Will saving throw, max spell level (cleric, druid, paladin and ranger), bonus spell (cleric, druid, paladin and ranger), spell strength (cleric, druid, paladin and ranger), conversation/dialogue options, and yet more skills

Charisma - max spell level (bard, sorcerer), bonus spell (bard, sorcerer), spell strength (bard, sorcerer), bard special abilities, conversation/ diaglogue options, and even more skills

Charisma affects also turning/rebuking undead and paladin's lay on hands.