Check out http://eschalon.wikia.com/wiki/Ability_Scores
. In short, you can pump any stat as high as you want and it will continue giving benefits.
In my opinion, Endurance (for max HP and HP regeneration) and Perception (for max MP and MP regeneration) are the best two places to spend points. Strength is helpful for carrying more stuff, and you obviously need Intelligence and/or Wisdom to be able to use your MP. I'm not a big fan of increasing Dex, Speed, or Concentration.
You're also probably better off picking a single combat skill to focus on rather than trying to balance piercing and bows.