This is not a Steam key, please do not activate it in Steam.
Once you receive your key, you can activate it in https://socialclub.rockstargames.com/activate. In order to do that, you must log in to an existing Rockstar Social Club account or create a new one.
After activating, you can download the game via the setup tool provided by Rockstar.
These are the predicted system requirements
OS: Windows XP, Vista or Windows 7
CPU: Intel Core 2 Duo 2.66 GHz or similar
RAM: 2 GB
HDD: 20 GB free disk space
Graphics: 512 MB Graphics Memory
Sound Card: DirectX 9 Compatible
DirectX: Version 9