As reported yesterday Microsoft is offering special huge discounts on Windows 7 Upgrades when you pre-order now. Amazon.com is now offering the Windows 7 Home Premium Upgrade for just $49.99, which amounts to 58% in savings.

Also available at a steep discount is the Microsoft Windows 7 Professional Upgrade selling for $99.99 instead of $199.99.

The Windows 7 Professional offers you Windows XP mode, Domain Join and automatic backup, which Windows Home Premium is not offering. So if you want to use Windows 7 for work then Windows 7 Professional is the better choice.
Windows 7 will be released on October 22nd. Amazon offers Release Date Delivery if you choose that option on check-out. All editions of Windows XP and Windows Vista qualify you to upgrade. As you know you have to do a clean install of Windows 7 on a Windows XP machine and cannot do an in-place upgrade as you can do on Vista.
More details on Amazon.com.