Well any extra hp is important! Believe me. In some battle i survived with only 2-3 hit points, man i had so much luck. Con is really important, especially for fighters. And the hp gain when youre higher than lv 9 is also important. I think fighters with const 18 get then 4 hp every lv up. Until lv 10 you will approximately have ~90 hp and bevore you will reach the exp cap, there are other ~30 hp (all depends on which class you have) waiting for you. More Information you can get at:
http://www.planetbaldursgate.com/bg2/
Just look at the tables and character information, you will find what you need, i hope