NWN is sorta BG3, but its a totally different storyline (if there is one?). There probly wont be a BG3 at any point in the future cuz i do believe you finish the story in ToB (am i right?, i've never played ToB..yet)
And there is nooo way your Celeron 466 can handle NWN, its goin totally 3d and needs a freakin spaceship comp to run it..