On the MSDN you get a single key which you can use with either the 32 bit or 64 bit version (but only one at a time), and I'm sure I've seen product descriptions where it states that you get both versions when you buy a disk. I can't find it right now to confirm that though.
You can't "upgrade" from the 32 bit to the 64 bit though as they are completely different architectures. So to move to 64 bit you have to do a complete fresh install.
I've found this link about Vista 32 bit vs 64 bit which confirms that they are completely different. It implies you get both on the installation disk (or two disks) but doesn't state it outright. Here's the same page for Windows 7
Ah - here we go - Amazon link:
Windows 7 Home Premium (includes 32-bit & 64-bit versions) makes it easy to create a home network and share all of your favorite photos, videos, and music--you can even watch, pause, rewind, and record TV
Single license, 1 installation