Upgrade to Cubase 6 - clean install

I currently run Cubase 5.5 on a 32-bit Windows XP system. I have purchased a new PC and will be moving to 64-bit Windows 7. I have also purchased an upgrade to Cubase 6.

Can I simply install the Cubase 6 upgrade and get that authorised via my USB key, or do I need to install a previous version (Cubase 5) for the upgrade to work?

Just 6 is fine.