Holy cow, I wrote a book!
Probably the biggest advantage of 64-bit Windows is not the larger registers
but rather the expansive 64-bit address space.
Recall that even when the /3GB switch is set,
32-bit programs receive only 2GB of address space unless they
indicate their willingness to cope with addresses above 2GB by
passing the /LARGEADDRESSAWARE flag.
This flag means the same thing on 64-bit Windows.
But since 64-bit Windows has a much larger address space available
it can afford to give the 32-bit Windows program the entire
4GB of address space to use.
This is mentioned almost incidentally in
Knowledge Base article Q889654
in the table "Comparison of memory and CPU limits
in the 32-bit and 64-bit versions of Windows".
In other words, certain categories of 32-bit programs
(namely, those tight on address space)
benefit from running on 64-bit Windows machine,
even though they aren't explicitly taking advantage of any 64-bit features.