Microsoft has posted download links to the ISOs to the Windows 8.1 Preview at
http://windows.microsoft.com/en-us/windows-8/preview-iso
The download is 2.5GB and 3.3GB for the 32 bit and 64 bit versions respectively.
Microsoft also has a FAQ detailing system requirements at http://windows.microsoft.com/en-us/windows-8/preview-faq
Engadget has a detailed overview of the Windows 8.1 changes from Windows 8.
