To download the Windows 8 Developer Preview, simply head over to the Windows Dev Center
and choose whether you want a 32 or 64 bit version.
There's also a 64-bit Windows Developer Preview with developer tools available for download, which also comes with the Windows SDK for Metro style apps, Microsoft Visual Studio 11 Express for Windows Developer Preview, Microsoft Expression Blend 5 Developer Preview and 28 Metro style apps.
The recommended Windows 8 system requirements are as follows:
1GHz or faster 32-bit or 64-bit processor
1GB RAM (32-bit) or 2GB RAM (64-bit)
16GB hard disk space (32-bit) or 20GB (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver
A screen that supports multi-touch if you want to use touch input
Do note that you can't uninstall the Windows 8 Developer Preview, so you'll need your Windows 7 disc to hand should you want to revert.
The Windows Developer Preview with developer tools requires a clean install, but if you opt for the one of the versions of Windows 8 without the developer tools and install it on a machine running Windows Vista or Windows 7, you can keep accounts, files, and settings. You can also install one of the two versions on Windows XP and keep your accounts and files (but not settings).
Windows 8 comes as an .iso image, so you'll need to burn it to a disc and boot from that to begin the installation.