Here are the system requirements for the Windows Phone 8 SDK
If you have already been developing for Windows Phone 7 note the new requirements
1. You need to be running a 64 bit Windows 8 OS to install the Windows Phone 8 SDK.
2. If you don’t meet the requirements for the Windows Phone 8 Emulator, the Windows Phone SDK 8.0 will install and run but the Windows Phone 8 Emulator will not function and you will not be able to deploy or test apps on the Emulator. see http://blogs.msdn.com/b/uk_faculty_connection/archive/2012/10/24/hyper-v-list-of-slat-capable-cpus-for-hosts.aspx for details on SLAT
The emulator uses Hyper-V under SLAT so if you try to run a project in the emulator and Hyper-V is not enabled, you will be prompted to turn on Hyper-V. Turning on Hyper-V will require you to restart your computer.
If you visit the Windows Phone Dev Center you’ll find all documentation and samples for the Windows Phone 8 SDK.
Here are a couple of new features here just to whet your appetite
No. Apps built for Windows Phone 7.5 still run on Windows Phone 8, so finish and publish the apps using your free developer account from http://create.msdn.com via http://www.DreamSpark.com
If you want to leverage some of the new Windows 8 features, you can do that in your next release.
Check out the following videos from Build 2012 on Windows Phone 8 Development