If you want to play the PC version of Batman: Arkham Knight, you're going to need a powerful Bat-computer. Even the minimum system requirements are pretty substantial.
To get the game running at the lowest settings, you'll need six gigs of RAM and a 2GB video card. That's a big step-up from Batman: Arkham Origins, which you could play with 2GB of RAM and a video card from 2007. Knight also requires about three times as much hard drive space as Origins.
The system requirements listed on NVIDIA's website are broken down into three categories: minimum, recommended and ultra. Here's the full list:
Minimum System Requirements
Because the specs were published by NVIDIA, they conveniently didn't mention video cards made by rival AMD. It's pretty easy to figure out what the AMD equivalent for each level is. A minimum would probably be something like an HD 7870 or R9-270X. The HD 7950 and R9-290X are likely good choices for recommended and ultra, respectively.
While running the PC version at ultra settings is going to give you the best-looking Arkham Knight experience possible, it sounds like the console versions won't be too shabby, either. The PS4 version, for example, is said to run at 1080p. Rocksteady didn't have to spend any time making PS3 or Xbox 360 versions so they can focus on optimizing the game for newer hardware and pushing the visuals.