IMHO, the right solution is to make the 64-bit edition the default
download
and to work on making the error message people get when trying to install it
on a 32-bit machine nicer: "We're sorry, but your computer is too old to
install this 64-bit version of Fedora. Please download the legacy 32-bit
edition instead."
That would be an aweful first experience. Imagine her thoughts: "Oh. I
made something wrong.. Maybe it's nothing for me.."
I suggest to let it as it is and check the system (if 64bit or 32bit)
when it is running.
Maybe then a little popup comes up with something like:
"You're pc could be run faster, if you upgrade this operating system
to the 64bit version of it. You can download them here if you like:
[Link]"
Even better would be:
"You're pc could be run faster, if you upgrade this operating system
to the 64bit version of it. You can do so by clicking here: [Button]"