There's nothing to blame on the game. It isn't NWN's or Biowares fault that your hardware is WAY out of date. If you have been following the released titles lately, and also played them, then you already should have much more decent system then you do now. But I guess that you don't have.
Which means that it's really about the time to go out and get a new hardware. NWN is one of the new titles coming out. And there will be much more, and all these new titles will not run on your system either. So, take it as a signal