Intel i5 or above, 8 GB or more memory, ATI 6870 or Nvidia 660 or above, if you have a 20/22 inch screen.
Also invest in a solid state hard drive. Those improve load times dramatically.
I built a system with those specs + mainboard and noise dampening enclosure for 750 US dollars. I already had a good monitor and peripherals so I saved some $ there.

This will play the game with all graphical settings at maximum and you will get an average 60 fps. If you are in an outdoor area with 16-24+ other characters on the screen it will dip to 20-35 fps.

As others have said, the game engine could be coded better so you will have stutters and a few hiccups here and there no matter how good your system is.