i'm used to bg2 and
at first i wanted to make a fighter4/mage x character (to have 3 points in bows) but then as i had barb 1/fighter 1/wizard 1 (i started barb then decided that fighter is better and tought to add the 3 left fighter levels later) i read: 6000 xp for next level!
does that mean that if you take 4 fighter levels in the beginning you are always 4 levels behind a wizard-only? i mean isnt that stupid? is there any sense in multiclassing now if i am always that much levels behind and get important spells that much later?

Character levels are fixed experience point increases which are equal to "current level * 1000." Say you have a level 1 human cleric. He gains 1000XP. You could make him a cleric 2, keeping your class and character levels conjoined. Or you could switch him to sorcerer for level 2...

He's now character level 2: Cleric 1/sorcerer 1

2000 more xp later you select fighter...

He's now character level 3: Fighter 1/Cleric 1/Sorcerer 1

3000 more xp later, you select Bard....

He's now character level 4: Bard 1/Fighter 1/Cleric 1/Sorcerer 1

Or you could have stuck with cleric and he'd be a Charcter Level 4, Cleric 4.