I am wondering if there is a way to have the dialog box be centered in the middle of the screen, or be freely moved. I am playing the game in 1920x1080 resolution, and it sucks to be staring at the top left corner of the screen every time I speak to an npc.

Not for the original NWN (NWN2 has a far more flexible UI with almost everything movable and sizable).

Look on the bright side though - if you had a 3-monitor setup, you'd be craning your neck to the left all the time.