Well, I was hoping I wouldn't need to reply in this thread once I installed my Seagate 500Gb (AS model) 7200rpm HD but I tried installing it yesterday and here are the results of the ordeal:
Removed old stock HD and installed new one (lost one screw in the process - reminder to self - buy a new screw). Once it was installed I just put the bottom cover in place but did not screw it together (I had a feeling I would need to put the stock HD back in).
Rebooted the machine with the Leopard DVD in and waited....once at the Select Volume screen, (no volumes present), I used Disk Utility to format the HD Mac OS Extended (Journaled). It seemed to format perfectly fine.
I now selected the newly formatted HD and the Leopard install began but seemed to stall after approx. 10% had installed. It never gave me a Estimated finish time but was stuck on "calculating".
After waiting another hour, it finally conked out and gave me the Install Failure screen. Tried one more time, fomatting then installing and still got an Install Failure.
I took out the new Seagate, jumpered it to SATA 1.5Gbps then reinstalled the drive remembering that many here have been successful with that method. It did work....sort of.
The OS installed fine and it then brought me to the screen where it asked if I wanted to restore from a time machine backup. I selected yes and picked a backup I had on my USB drive. The backup hung at the last second and never completed. It said less than one minute remaining and I waited over an hour. I finally gave up and pressed the power button to reboot.
As soon as I did that, it would not reboot into OSX but instead just gave me the folder with the X through it.
I gave up and re-installed the stock HD and here I am in the same boat as the rest of you. (sigh).