What maximum level is attainable with 8 mil. experience (for different classes i mean). I know Clerics reach level 40, but mages, figters and others?

Fighter, Barbarian - 40
Paladin, Ranger - 34
Mage, Sorcerer - 31
Cleric, Monk - 40
Druid - 31
Thief, Bard - 40
Fighter/Mage - 24/20
Fighter/Cleric - 24/25
Ranger/Cleric - 21/25
Fighter/Druid - 24/21
Fighter/Thief - 24/28
Mage/Cleric - 20/25
Mage/Thief - 20/28
Cleric/Thief - 25/28
Fighter/Mage/Cleric - 18/17/19
Fighter/Mage/Thief - 18/17/22

also worthy of mentioning is that you can remove the experience lvl cap with an xp lvl cap remover, it can be found right here at SP

The "safe" XP cap remover is better, allowing you to reach level 40 in both/all 3 classes if you are multi or dual classed. If you completely get rid of the XP cap, sometimes when you go over lvl 40, your characters end up being like lvl 1 characters again. I think this is because the engine doesn't support levels over lvl 40 or something.

So if you go over level 40 in all classes you'll start from level 1 again?

No, it means that if you're of any level that doesn't exist on the game's 2DA tables then you'll end up with the THAC0, saving throws etc. of a level one character by default. Apparently. I've never done it, and I'm not sure why anyone would make an XP cap remover that doesn't update all the other tables as well.