To answer one question, the EULA says for the Upgrade, you can no longer use the sacrificial OS.
But I do think making a image backup is a good idea, if for no other reason, a problem occurs with the Windows 8 install and you have to start over. You could then, delete all the current partitions during the install and not worry about recovering. You do have an OEM install with special partitions. You would need to include those in the image if you want to fully recover. If there was any way for you to test the image, however you make, it might be a good idea. If it was me, I would make the image, replace the hard drive and re-image the new drive and then do the install. That way you always have the original in case it is needed.
If you use the Windows Backup and Restore utility, you do need another drive to hold the image, either internal or external. Images are normally around 50-60% the size of your install, so it would need a fairly large flash drive. Backing up to a network location is also a possibility.
Right now you can get Windows Media Center free for supplying a e-mail address, or you can add that feature for $9.99, I believe. This would take care of your DVD situation.
Secure Boot needs a UEFI bios, and you probably would not use it anyway.