I'm nitpicking here, but this is NOT the Windows 8 Beta. It's a developer preview, which is pre-beta software.
Anyhow, I agree that a VM is the safest option.
I used a spare HDD I had lying around, plugged that in, set it as first HDD in the Bios, installed the Developers preview and checked it out, once finished, switched the HDD order in the bios again and i am happily back to windows 7!
Just use a virtual machine.
or just partition your hard drive... 20 gb should be enough and install windows 8 in that partition by the way just in case you dont know in this way you will dual boot that way you dont need to find another hard drive
The above suggestions are exactly what you should look into. IF you want to run it in a VM try the free Oracle virtualbox .
If You've never set-up a dual boot before, I would suggest running it in a VM first. If you like it, then go ahead and set up the dual boot. I set it up in a dual boot with windows 7 on an Aspire 9300 laptop, and after 20 minutes of using it, I was wishing I had just used a VM. But that's just me.