Well it looks like the regular/non-upgrade version of Win 7 x64 pro ISO download has the exact same name as the one I downloaded after purchase, called:
X15-65805.iso
So, unless someone knows otherwise, it looks like the ISO is the same regardless of version, and the actual key controls whether you have the full or upgrade version.
So, I'll be using the DigitalRiver ISO that comes with SP1, and see what happens when I try my upgrade key (after installing Vista first).