After Customer Preview, today Microsoft releases the last preview of Window 8. You can download from this link:
System Requirements:
Windows 8 Release Preview works great on the same hardware that powers Windows 7:
Processor: 1 gigahertz (GHz) or faster
RAM: 1 gigabyte (GB) (32-bit) or 2 GB (64-bit)
Hard disk space: 16 GB (32-bit) or 20 GB (64-bit)
Graphics card: Microsoft DirectX 9 graphics device with WDDM driver
Additional requirements to use certain features:
To use touch, you need a tablet or a monitor that supports multitouch.
To access the Windows Store and to download and run apps, you need an active Internet connection and a screen resolution of at least 1024 x 768.
To snap apps, you need a screen resolution of at least 1366 x 768.
Internet access (ISP fees might apply)
"Important: If you decide to go back to your previous operating system, you'll need to reinstall it from the recovery or installation media that came with your PC, which is typically DVD media. If you don’t have recovery media, you might be able to create it from a recovery partition on your PC using software provided by your PC manufacturer. Check the support section of your PC manufacturer’s website for more information. After you install Windows 8, you won’t be able to use the recovery partition on your PC to go back to your previous version of Windows. "
More info at Microsoft site.