From March 5th, you will be able to pre-order one of 3 variants of ArmA 3, all of which will give you access to the Alpha.
The Alpha version is invite-only will have the following features:
Island of Stratis (20 kilometers squared) 3 Factions 8 Vehicles 12 Weapons Weapon Attachments Customizable Gear 4 Singleplayer showcases (“Infantry, Vehicles, SCUBA and Helicopter”) Scenario Editor (Limited)
If you purchase one of the actual packs (Alpha, Digital Deluxe Edition or Supporter Edition), you get Alpha keys to give to friends. You also get 2 Multiplayer Scenarios and Modding capability.
Additionally, the minimum and recommended requirements to run ArmA 3 have been added:
MINIMUM OS Windows Vista SP2 Windows 7 SP1 PROCESSOR Intel Dual-Core 2.4 GHz AMD Dual-Core Athlon 2.5 GHz GRAPHICS NVIDIA GeForce 8800GT ATI Radeon HD 3830 Intel HD Graphics 4000 GPU MEMORY 512 MB DirectX® 10 RAM 2 GB HARD DRIVE 15 GB free space HARD DRIVE (ALPHA) 10 GB free space AUDIO DirectX® compatible on-board OTHER Internet connection and free Steam account to activate
RECOMMENDED OS Windows Vista SP2 Windows 7 SP1 PROCESSOR Intel Core i5-2300 AMD Phenom II X4 940 GRAPHICS Nvidia GeForce GTS 560 AMD Radeon HD 7750 GPU MEMORY 1 GB DirectX® 11 RAM 4 GB HARD DRIVE 25 GB free space HARD DRIVE (ALPHA) 20 GB free space AUDIO DirectX® compatible soundcard OTHER Internet connection and free Steam account to activate
^You can either pre-purchase the full game and get access to the full Alpha and Beta, or; if you are a VG journalist etc. you may have been lucky to have been invited to play the Alpha-Lite version, which features no Co-op Missions or modifiable content, for free without having to pay.