So many questions
Right, the retail upgrade version was designed to allow owners of older OS's to get a cheap way of upgrading to the latest OS.
So if you've bought WinXP you can upgrade this license to a Vista license for less than buying the full version.
A Vista license will cover you for both the 32bit & 64bit version of the OS.
However Vista Home Premium only ever shipped with the 32bit install media - to get the 64bit installation media you needed to send off £7 to Microsoft who would then send you the 64bit media.
It used to be the case that you could install your retail upgrade as a full version and simply insert your old OS disc when requested.
Example being you bought WinXP Upgrade.
You could have an empty HD and during the WinXP installation you would be asked to insert say your Windows 2000 CD and this verified you had an OS to upgrade from.
This changed with Vista - probably because simply inserting an old OS disc in no way proved you held a license for said OS.
I believe there are ways around it, but generally speaking to use the upgrade you do need to have the old OS installed first.
However, as you are moving from a 32bit to a 64bit OS your old installation will in effect be removed during the VIsta installation.
Are you safe buying second hand?
Make sure you get the full retail packaging with the CD key and that the disc supplied is legitimate and not just a CDR.
The onus is on the seller to have removed the OS from his own machine befroe selling, so from this point of view you'll be fine.
However if he has kept a note of the installation key and keeps on using it then it "may" cause some grief when you come to activate your OS if it doesn't work automatically online.
However you should be able to talk your way through that.
There is always potential for problems with second hand software.