I'm playing an elf, with ranger/rogue class. I was told by my friend who is familiar with 3rd edition rules that if you dual class and one of the classes is not a preferred class of that race you are penalized 20% of experience points as a result unless you have classes within at least 2 levels of each other. For example, the preferred class of an elf is a magic user. I am using the ranger/rogue combination, so if I decide to level up as a level 4 ranger, and a level 1 rogue I will get penalized 20% of experience after all my battles etc. etc. Is this true in NWN????

Yep, thats the case. One of the reasons I chose human for my first character, but now I think I leveled up way to fast as I am a total of 15 levels at the end of chapter two. Dont worry if have the exp restriction, if you do a majority of the sidequests you'll have exp to spare, or at least it seems that way to me as far as I've gotten in the game, maybe someone who has finished the game could elaborate more on this...