You can't legally have two simultaneous installs with the same product key. You will have to uninstall the old 32 bit version in order to legally install the 64 bit version.
To more directly answer your question, you need the media regardless. You will have to download the English x64 .iso and burn it. You can get it directly from Microsoft, and someone has already taken the opportunity to link all the .iso files on one site:
http://www.mydigitallife.info/official-windows-7-sp1-iso-from-digital-river/
digitalrivercontent.net is owned and hosted by Microsoft, so feel free to download and use those .iso files.