System Requirements for Unity Authoring
Windows: XP SP2 or later; Mac OS X: Intel CPU & "Leopard" 10.5 or later. Note that Unity was not tested on server versions of Windows and OS X.
Graphics card with 64 MB of VRAM and pixel shaders or 4 texture units. Any card made in this millennium should work.
Using Occlusion Culling requires GPU with Occlusion Query support (some Intel GPUs do not support that).
The rest only depends on the complexity of your projects!.
System Requirements for Unity iOS Authoring
An Intel-based Mac
Mac OS X "Snow Leopard" 10.6 or later
The rest only depends on the complexity of your projects!
System Requirements for Unity Android Authoring
In addition to the general system requirements for Unity Authoring
Windows XP SP2 or later; Mac OS 10.5.8 or later
Android SDK and Java Development Kit (JDK)
System Requirements for Unity-Authored Content
Windows 2000 or later; Mac OS X 10.4 or later.
Pretty much any 3D graphics card, depending on complexity.
Online games run on all browsers, including IE, Firefox, Safari, and Chrome, among others.
Android authored content requires devices equipped with:
Android OS 2.0 or later
Device powered by an ARMv7 (Cortex family) CPU
GPU support for OpenGLES 2.0 is recommended