Nothing special.
Leave the old hdd unplugged and then install Windows 7 on your new HDD.
Once Windows 7 is installed, plug in the old HDD, ensure that the new HDD is set as the primary boot device in system BIOS, and reboot. You'll be good to go. Your new Windows 7 installation will assign the next-available drive letter to your old HDD.
Leave the old hdd unplugged and then install Windows 7 on your new HDD.
Once Windows 7 is installed, plug in the old HDD, ensure that the new HDD is set as the primary boot device in system BIOS, and reboot. You'll be good to go. Your new Windows 7 installation will assign the next-available drive letter to your old HDD.