Windows 7 is available in six different editions, but only Home Premium, Professional, and Ultimate are widely available at retail. The other editions focus on other markets, such as the developing world or enterprise use. All editions support the 32-bit (IA-32) processor architecture and all editions except Starter support the 64-bit (x86-64) processor architecture (64-bit installation media is not included in Home Basic edition, but can be obtained from Microsoft).
If you go to the website support I think there should be a place to order the 64 bit DVD well I think its like 9.99 or close and they say thats for shipping. As long as its retail not OEM.
I did this not to long ago for Vista so I would beleive its the same for win 7.
This may help may not. http://support.microsoft.com/kb/326246