memory - What's a good rule-of-thumb for choosing between Windows 32 bit and 64 bit?

I'm installing Windows on an old(ish) machine again - and am again faced with the question of whether to install the 32-bit or 64-bit version.

Instead of asking this question specifically for some individual setup (like here), let me ask more generally:

What is a good rule of thumb, or combination of criteria or rules, for deciding between 32-bit and 64-bit?

Possible factors affecting the decision:

• Which version of Windows is to be installed (Win XP, Win 7, Win 8.1, Win 10); I'd guess the newer the version, the better it is to go 64-bit.


• Memory size (1, 2, 4, 8, more) - 32-bit prevents you from benefiting from > 3GB memory or so; and 64-bit must eat up more memory.


• CPU speed - is this at all a factor?



• Others, I'm sure...