No, you cannot upgrade from a 32bit oem to 64bit oem. I had several
conversations with MS about this. The license terms you mention below
refer to Vista Ultimate which gives you the ability to install any 32bit
or 64bit version of Vista that you'd like as long as only 1 is installed
at a time. Obviously this means that you can upgrade from 32bit premium
oem to 64bit Ultimate.

Riggs wrote:
> Hi
> I have got a new toshiba laptop with an AMD Turion64 x2 cpu and 2gb Ram
> vista home Premium 32bit OEM.
> I would like to know i can install a 64bit version of vista using using my
> same product key i do understand i would have to unistall my 32bit version.
>
> MICROSOFT SOFTWARE LICENSE TERMS
> c. Alternative Versions. The software may include more than one version,
> such as 32-bit and
> 64-bit. You may use only one version at one time.
>
>