If you like D&D based RPGs, I recommend Neverwinter Nights 2 plus the Mask of the Betrayer expansion. The original campaign ain't bad, and MotB is worth the price of admission alone (especially if you like stuff written by Chris Avellone, like Planescape: Torment). NWN2 is a popular thing to hate because it was horridly buggy on release, but much of that has been fixed. If the general set-up/description sounds good to you, I'd encourage giving it a try.
Plus the nice thing about NWN2 is that once you're done with the official stuff, you can download user-made modules, and by now there's some nice stuff out for it. It's potentially a source of endless gameplay.
The last game _I_ got for PC and thought it was worth buying was Fallout 3, but you've played that.
No, I lie. The last game I got for PC was the demo of Crayon Physics, which was certainly worth the free cost, and I will probably put down the $19.99 for the full version when I have time. If you like physics and puzzles and want to do something totally different than action-rpg-strategy.... then absolutely, go for that.