^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.