TTBOMK, the stat levels at which an ability changes are:1 3 5 7 9 11 13 15 17 19 21 25 30 35 40 50 75 100 & 125
A stat of 1 gives you an ability of 0, a stat of 3 gives you an ability of 2, and so on up to an ability of 18.
So once you get a stat above 50, it's not worth raising it much more unless you intend to use any of the relics that drop stats a lot.
This also demonstrates why I have no time for items "of the Gods". By the time you find them, your stats will already probably be at a level where +10 is going to be of little if any use in actually raising abilities.
These ability levels govern things like hit points, the amount of damage you do, how likely you are to hit, etc.
As I understand it, though, spells are governed by skill levels, although I don't think the relationship is as simple as the books imply - for some spells at least.
