Microsoft gave out the system requirements for Windows 10 at WinHEC.
For the desktop version of Windows 10, the system requirements are almost identical to Windows 8.1. For a 32-bit OS, you will need 1 GB of RAM with 16 GB of storage space, and for a 64-bit OS, you will need 2 GB of RAM with 20 GB of storage space.
Regardless of whether you choose 32-bit or 64-bit, you will need a GPU supporting DirectX 9 and an 800 x 600 resolution display. The display for consumer variants of the OS must be 8 inches or greater in size, and the Pro variants must be 7 inches or greater in size.
The only change between Windows 8.1 and Windows 10 for system requirements on the desktop is the mandate of UEFI v2.3.1. Windows 8.1 was required to have UEFI v2.3.1 for Secure Boot, but this was an optional feature. In Windows 10 this is no longer optional.
Smartphones have a different list of requirements for Windows 10. The minimum amount of RAM is 512 MB, but will ultimately depend on the resolution of your smart device. 512 MB is the minimum for running at 854 x 480 or 800 x 480 resolutions, but with a resolution of 2560 x 2048, you will need at least 4 GB of RAM.
At least 4 GB of storage space is required. If your smartphone only has 4 GB of storage, then an SD card will be required to store additional updates.
A DirectX9-compatible GPU is still a requirement, but screen size can be between 3 inches and 7.99 inches; Microsoft will likely view devices larger than that as needing the desktop OS.
A few SoCs have been confirmed to run Windows 10 already, including Intel's Cherry Trail, Atom x3, and SoCs based on Skylake; AMD's Carrizo and Carrizo-L; and five Qualcomm SoCs (MSM8909, MSM8208, MSM8994, MSM8992 and MSM8952). Some smartphone makers such as Xiaomi are already testing Windows 10 for use on their products.