P.S. I am not looking to build a computer.
If you build your own from Newegg components for example you will end up with a really powerful machine with less than $1000
If you don't want to build your own, just buy an inexpensive desktop with quad core CPU and replace the video card with one of the following:
1 GB AMD 5770
1 GB NVIDIA GTX 460
1 GB NVIDIA GTS 450
Those are the three cheapest DX11 cards that can handle most games at 1920x1080 or 1920x1200, price ranges from $110-150. just make sure whichever desktop you buy has PCI-e slot and enough internal space for the GPU.
Here are more video cards you might consider depending on your budget:
http://www.tomshardware.com/reviews/best-graphics-card-game-performance-radeon-hd-6670,2935-2.html
http://www.tomshardware.com/reviews/best-graphics-card-game-performance-radeon-hd-6670,2935-3.html
Examples of gameplay or screens with graphics mods:
http://www.youtube.com/watch?v=SHjC6_61kg4&feature=youtu.be&hd=1
http://youtu.be/sndIC1bK4CY?hd=1
http://youtu.be/Z4G0de770YA?hd=1
http://youtu.be/5l1jZRbkl0k?hd=1
http://youtu.be/SHjC6_61kg4?hd=1
http://youtu.be/as6aqZHDYRU?hd=1
http://wolf3d.darkbb.com/t2037-oblivion-mods-to-make-oblivion-look-better