If you're running a 32-bit version of Windows Vista, you can only upgrade to a 32-bit version of Windows 7, and if you are current using a 64-bit version of Windows Vista, you can only upgrade to a 64-bit version of Windows 7. To switch from a 32-bit version of Vista to a 64-bit version of Windows 7, use the Custom installation option.

• Windows 7 comes in both 32-bit and 64-bit processor versions
• 64-bit version of Windows 7
• Can support up to 192 GB of RAM
• Has a greater theoretical limit for processing data
• Not all software and hardware is compatible with the 32-bit and 64-bit editions
• Windows 7 Upgrade Advisor

http://windows.microsoft.com/en-us/windows/downloads/upgrade-advisor

By downloading, installing, and running the Upgrade Advisor, you can create an easy-to-read report that lists all system and device compatibility issues with your current computer. The Upgrade Advisor recommends ways to resolve any issues it uncovers.It helps you choose the correct version of Windows 7 to meet your needs.

• Utility available from Microsoft to analyze the suitability of a computer to run Windows 7

The Windows 7 Compatibility Center is a Web site that lists thousands of hardware and software products and their compatibility with Windows 7. Each product is separately documented for its compatibility with the 32-bit and 64-bit editions. If the product is not compatible there may be a recommended action listed that will tell the user what to do about it, such as upgrade the application or hardware that they are researching. Links to manufacturer Web sites are provided if updates, downloads, or special instructions are available to the user.