Hey #1newb--I'm another newbie, and installing Mandrake 9.2 now. The folks here are very helpful, of course--they've solved lots of problems for me--but if you don't have the manuals, you can download the 9.1 manuals free here:
I don't think they're very different from the 9.2 manuals, and they have a good description of the install process. Marcel Gagne's Moving to Linux
is good too.
As Peacedog said, back up just in case (though I didn't have any problems related to data loss). As long as you've got the new hard drive properly installed, you should be able to pop in Mandrake Install Disk 1, and during the install process, it should find the hard drive, and if you choose that drive to install it on, it'll format it for you.