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Be sure to pick Windows/Windows 8 for your VM. If you're going to be testing out the 64-bit version, select Windows 8 (64bit).
VirtualBox may think that you'll only need 1GB for 32-bit Windows 8. Don't believe it. Set it up with at least 2GBs—that's 2,048MB.
Next create a new virtual drive for you Windows 8 VM. 20GBs should be more than enough for test purposes.