Hitpoints

• Up to level 9 hit dice + con modifier
• max hit dice is 1d10, con modifier 4
• So, a Fighter can get a maximum of 14 hp per level up to level 9. From thereon, he gets a fixed 3
• HP per level.

Hitpoints dual class

Dual classes gain no HP as long as the first class is inactive. Example: Fighter levels up to 13 as Fighter, then dual classes to Thief. The Fighter class becomes inactive until the Thief reaches level 13 again. The Thief gets NO hitpoints for his first 13 levelups, but beginning with level 14 he gains 2 hp per level

Ability scores (stat points) possible boni:

In BG1 there are tomes that give +1 to one stat points. Six such tomes exist, one for strength, dex, con and so on.

In BG2, you inevitably loose one point in your primary stat during one of your dreams

The Machine of Lum in Watchers Keep Level 4 allows to gain one point in each stat. You can take them all for your own char or let party members get something. For example, it’s a way to get 18 int for Aerie using the Machine of Lum and the golden Ioun stone (+1 int) to push her int to 18 so she can learn level 9 spells.

The hell trials in Chapter 7 of SoA can give +2 to con, +2 to str and +1 to wisdom and charisma, depending on the options you choose. It can result in evil alignment.

Deck of Many things (Watchers Keep Level 4) can give you +1 to your primary stat and +10 HP.