Fixer, could you state a source that would support your statement about RAM modules that can only handle a 32-bit OS even if the processor supports 64? I have never heard of this limitation of RAM before; I would like to read more about it. To RAM and hard drives it should merely be a matter of reading and writing bytes of information and I cannot understand how either could be dependent on the OS bit-size.
Also, I know for certain that there is no difference between a 32 and 64 bit product key. And if you are referring to a W7 upgrade license then yes, a 32-bit XP or Vista license does qualify you to legally upgrade to a 64-bit W7, but it requires a custom install. As for changing from W7 32-bit to 64-bit on a machine that shipped with a W7 OEM license, there may be some question as to the legality of it, but reinstalling the 64-bit OS will definitely work, if the hardware supports 64-bit, with the same OEM product key ... but you are correct it would require 64-bit drivers which may or may not be available..
A 32-bit system can only address about 3.3GB, not the entire 4GB. There have been some tests tweaking the system to allow full addressing but it is not functional with all software and, though it may be fun to play with on a spare test machine, I would not recommend it for standard use.
Almost all machines that shipped with windows XP were 32-bit, only a very few ran XP Pro 64, and a good percentage of Vista machines shipped with 32-bit. Even W7 shipped as 32-bit on many machines. With the exception of netbooks and computers with under 2GB of RAM, most of the XP and nearly all of the Vista & W7 computers can easily run 64-bit. I would have to strongly disagree with your statement and think it would be a huge disadvantage to limit your upgrade to 32-bit simply based on what bit-size was originally installed.