where did you Download the win7 image from if it's from an MS online store you should be able to download either version or both the fact of the matter is the only difference between the downloaded version and the retail is a box and a couple of disks if you purchase 1 you get both versions
I downloaded the RTM (final) Windows 7 from Microsoft (TechNet Plus account) and had to make a choice whether I wanted 32 bit or 64 bit at the download site. You can download either or both and the same installation key applies to either version. Retail site downloads may be somewhat different. Perhaps you could re-visit the download site and check the site very carefully. You likely would have the opportunity to download the 64 bit version and use your existing installation key to wipe and install the 64 bit version. You are actually buying the installation key, not the DVD containing the OS itself. (Kinda like they give you the car, but you have to buy the key to get it started.)
With the 64 bit version being slightly over 3 GB and the 32 bit version being slightly under 3 GB, separate DVDs would be required unless you use double layer format. Highly unlikely that the download for retail versions is 6GB in size. However hidden it may have been, I believe there was the opportunity to choose between either or both (one at a time) at the download site.