Nvidia listed the Batman: Arkham Knight system requirements today, and here is what you will need to play the game.
According to Nvidia, at minimum you will need an Intel Core i5-750 or AMD Phenom II X4 965 CPU, 6 GB of RAM, an Nvidia GeForce GTX 660 GPU with 2 GB vRAM, and 45 GB of hard drive space. Nvidia didn't list an AMD card, but the AMD Radeon HD 7870 is similar in performance and should work in place of a GTX 660.
The biggest change between the minimum system requirements and the recommended system requirements is the need for an Intel Core i7-3770 or AMD FX 8350 processor. The rest of the system requirements see smaller increases: 8 GB of RAM, an Nvidia GTX 760 GPU with 3 GB of vRAM, and 55 GB of hard drive space. Again, a comparable AMD card wasn't mentioned, but it is likely that the AMD Radeon R9 280 would be the closest GPU in AMD's lineup and should work fine as well.
"Ultra" system requirements, or presumably the requirements to run the game with maxed out settings, were also given, but the only change here is Nvidia's recommendation of an Nvidia GeForce GTX 980. The AMD R9 290x should work, too.
Regardless of what hardware you have, you will also need a 64-bit version of Windows 7 SP1 or Windows 8.1, and a broadband Internet connection.
Looking at these requirements, most modern computers that have suitable graphics cards will be able to play the game. The minimum CPU requirements are quite old anymore, and modern Intel Core i3 processors or quad-core AMD processors should do the job. Light gamers might be out of luck, however, as they are less likely to have a GPU capable of handling the game.