The system requirements for Visual Studio 2015 preview are the same as in Visual Studio 2013.
Operating systems that support Visual Studio preview:
Windows 8.1 (x86 and x64)
Windows 8 (x86 and x64)
Windows 7 SP1 (x86 and x64)
Windows Server R2 (x64)
Windows Server (x64)
Windows Server R2 SP1 (x64)
The current installation of VS2015 preview must be installed in these operating systems (or installed on newer systems).
Hardware requirements
1.6 GHz or faster processor
1 GB RAM (1.5 GB if running on a virtual machine)
GB of free hard disk space
5400 RPM Hard Drive
DirectX 9-compatible video card with a display resolution of 1024x768 x 768 or higher
Other requirements:
Internet Explorer 10
Requires KB2883200 (available through Windows update)
For Windows Phone Development:
Windows Phone 8.0 development requires Windows 8.1 (x64) or later
Windows Phone 8.1 development requires Windows 8.1 (x86) or later
For Windows Phone Emulator, Windows 8.1 (x64) version Professional or later, and processors that support client-side Hyper-V and two-level address translation (SLAT)
vs2015 Preview Download page
VS2013 System Requirements
Visual Studio 2015 preview system requirements