it's better that you do. it's always ideal to reformat after a major upgrade. That way you'll get rid of any drivers that won't be used anymore and avoid any conflicts between them and the new ones. also, you might want to check your windows licence. an OEM copy might give you trouble if you upgrade the motherboard.
Anytime you change your motherboard (not flashing the bios) your OS installs a lot of system device drivers. Unless you know how to remove the previous ones, it's best to start with a clean OS to avoid system conflicts. Also if your hd is old, its best to replace it too.
So yes it is best to format your drive, but if you had a lot of files that you're trying to save get a new drive and use the old as a backup.