I'm getting a headache just looking at the available options:

Windows 7 Home Premium Full
      
Windows 7 Professional Full
     
Windows 7 Ultimate Full
   
Windows 7 Home Premium Upgrade
        
Windows 7 Professional Upgrade
      
Windows 7 Ultimate Upgrade
      
Windows 7 Home Premium Upgrade Family Pack

Microsoft Windows 7 Home Premium 32/64-bit (Upgrade from Windows 7 Starter
    
Windows 7 Professional 32/64-bit (Upgrade from Windows 7 Home Premium)
    
Windows 7 Ultimate 32/64-bit (Upgrade from Windows 7 Home Premium)

If you are ready to take the plunge, be sure to download the windows 7 adviser to see if your setup is worthy

http://www.microsoft.com/downloads/details.aspx?FamilyID=1b544e90-7659-4bd9-9e51-2497c146af15&displaylang=en