What is a great PC RPG game that is relatively open ended?

Im looking into getting a good PC RPG game. I have Morrowind GOTY but that gets very repetitive towards the end. Im looking into Fable but they say that only has about a 12 hour story mode. I also cant have anything to taxing on my computer. Oblivion and Two Worlds are even to much. So yeah. I need to know of a good open ended RPG or a good story based game with an extremely long story.