Just to clear something up, "x64" that was know it as is really an abbreviation. It's full designation is "x86-64" since it just extends Intel's "x86" IP. Thus one could make the case that when people refer to nVidia making a "x86" processor, they are referring to x86 and its extensions thus including x86-64 or x64.
Personally, if they are making a processor, given nVidia's tendency to go big or go home, I would be surprised if it's not x86-64 compliant.
However, There is still the issue of getting a x86 license from Intel and a "x86-64" license from AMD...