Xenovant, I tried lots of default creatures, and most of them cause lagging, save the ones Inayiti has mentioned.
MudEye, thanks for the idea, but I'm pretty sure no one else will have the lag issue on their end because they don't have an ATI 3870, like I do. It is not a matter of the module, nor probably NWN itself, but the video card. As I explained in the very first post, I had never experienced any problem with my game, till I bought this video card.
Thanks everyone, I'm just tired of this... been probably more than a year of trying things out, when the answer was clear from the beginning: Ditch the crappy card, and buy a new one. Sadly, I have no money to feed myself anymore, much less to buy a new video card.
I give up.