I have read this several times about Steam also. I would try it.
There is no harm in attempting to boot up your old HD once you replace the mobo. Sometimes Windows 7 will start since it has all of the needed drivers
in it's driverstore - it is only the order in which drivers fail to load which can stall it at boot animation.
Even then, you can boot Paragon Adaptive Restore CD to remove all drivers from the HD and it will start on new mobo. Or reimage with Acronis 10+ with Universal Restore.
I would unplug the data drive during any such operations including clean install.
Tips for a purrfect reinstall: re-install windows 7