Formatting a used MBP after booting from OS DVD? I'm installing a MacBookPro via DVD which I booted from. Do I use Disk Utility to format the HD which has an OLD OS X, with users info on it that I don't want, so it WON'T upgrade and retain user data, and instead would actually install a fresh copy with NOTHING inherited?
Yes, during install you can start disk utility and re-partition/format the hard drive.  Make sure you set it as a GUID partition type so that OS X recognises it is a boot drive.