The requirements for 32-bit vs. 64-bit Vista are completely wonky. They
said 512MB is good enough for the 32-bit PC, but on a 64-bit you'll
need 2GB, i.e. just to hold the double-width 64-bit data units. 2GB is
not double of 512MB, it's quadruple!
First of all even if there are 64-bit data structures in the 64-bit
version, the AMD64 ISA defaults to 32-bit data structures for
everything except address offsets. So at worst, I'd say a 64-bit
version of a program shouldn't be much more than 10% larger than the
equivalent 32-bit port.
Second, why does one require four times the memory of the other
version? Are they fudging about the requirements of the 64-bit
version, by overestimating it? Or are they severely crippling the
32-bit version to make it work on the smaller hardware?