^Basically what this guy said. Well there might be another way to do it but I have never tried doing it from 32bit to 64bit.
You can run the Windows 7 install and when you get to the part where you can format, just select the drive and select next instead of formatting. It should tell you it found another Windows installation on the drive and will place it into a folder called "Windows.Old" and install Windows 7 around it.
Still its almost the same as backing up and formatting then to do a clean install.
It does suck that there is no 100% way to go from 32bit to 64bit though.
You can use the Upgrade option during Windows 7 installation, which preserves your files, settings, and programs, only if you're currently running a 32-bit version of Windows Vista and you want to upgrade to the 32-bit version of Windows 7. Similarly, if you are running a 64-bit version of Windows Vista, you can only perform an upgrade to a 64-bit version of Windows 7. For more information, go to Upgrading to Windows 7: frequently asked questions on the Windows website.
If you want to move from a 32-bit version of Windows to a 64-bit version of Windows 7 or vice versa, you'll need to back up your files and choose the Custom option during Windows 7 installation. Then, you'll need to restore your files and reinstall your programs. For more information about performing a custom installation, see Installing and reinstalling Windows 7.