I know a high Dexterity gives you a better chance to hit. But supposing i want to make an archer, what is the crucial measurement, if there is one, that powers up his shot?

i think its rangers that need a real high wisdom and constitution to

Make an elven archer. Elves get 19 dexterity, which is a big bonus w/bows, and they also get a bonus w/bows and swords anyway (so that's more bonuses). I recommend a grand mastery in Shortbow for SoA, but Crossbow is also good, and it's definitely preferable if you're planning on playing through Throne of Bhaal. (This doesn't count as a spoiler, I don't think.)