The Windows 7 license for your factory OEM version allows the use of either 32 bit or 64 bit.
You only need to find a 64 bit installer.
All of the installers are the same except they have certain versions hidden due to an ei.cfg file in Sources folder. If you extract the ISO to the desktop and remove that file, you can recompile into iSO and burn to DVD using this tutorial: Bootable ISO - Create from Installation Files
You might try going back to the store and firmly but politely telling the manager what the salesman told you which was untrue, then insisting they provide you with a 64 bit installer.