If you're having both drives installed and connected at the same time off the same ATA chain, leave the 40GB as Master and set the new 80GB drive to Slave or Cable Select if it's being connected via the same ATA cable. Slave or Cable Select drives can be used as boot volumes just like Master set drives. If you're removing your 40GB drive, then yes, set the new one to Master. Once the new drive is installed and setup correctly, hardware-wise, simply boot from the Tiger v10.4 DVD and go into Disk Utility. Select the new drive from the right and format it as "Mac OS Extended (Journaled)." Then run the Installer. The new drive needs to be formatted as such first, otherwise the Installer will not see it as valid drive to install upon. (or see the drive at all, at that.)
FYI- sometimes after formatting a new hard drive, you'll have to restart your Mac first before the Installer will see the volume as a valid drive. So, after you format, and the Installer still doesn't see the drive, simply restart your computer and run the Installer again off the Tiger v10.4 DVD.
Hope that helps.
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