According to the list released by Nvidia, it appears that Batman: Arkham Knight will be quite a demanding game, as the minimum system requirements start with Intel Core i5-750 or AMD Phenom II X4 965 CPU, 6GB of RAM, Nvidia Geforce GTX 660 with 2GB of VRAM and 45GB of HDD space.
The recommended system requirements push the CPU up to Intel Core i7-3770 or AMD FX-8350, 8GB of RAM and Nvidia's Geforce GTX 760 3GB graphics card while HDD space is pushed up to 55GB. The ultra PC system requirements are a completely different story as while the CPUs are the same as on recommended, graphics card of choice should be at least a Geforce GTX 980.
It is not clear why the HDD space for both recommended and ultra system requirements is much bigger than minimum, but best guess is that we might see some high-resolution packs. Bear in mind that you will have to have a 64-bit Windows 7 or Windows 8.1 OS in order to run the game.
As you probably already know, Batman: Arkham Knight is scheduled to launch on PC, Xbox One and Playstation 4 on June 23rd.