I have an old Vista box sitting around, and I'm thinking of upgrading it to Windows 7 64-bit, as it has 4gb of RAM. Well, I know that there's an "in-place" upgrade from Vista, but does it work from 32-bit to 64-bit?
Windows 32bit to 64bit has never been supported.
If you are a fan and trust upgrading, you may want to take a look at "Windows Easy Transfer". This should allow you to move your accounts, documents, settings and a whole lot more to a temporary location whilst you can do a full install of the new Windows, then move all your stuff back.
Can I upgrade from a 32-bit version of Windows to a 64-bit version of Windows?
No. If you are currently running a 32-bit version of Windows, you can only perform an upgrade to another 32-bit version of Windows. Similarly, if you are running a 64-bit version of Windows Vista, you can only perform an upgrade to another 64-bit version of Windows Vista.
(this is from their Vista page, I cannot find it for 7, however I am 99.9999% sure it is the same!
Check this answer from Microsoft:
RickH's link refers to a custom install.
While you cannot "upgrade" (as in an inplace upgrade) from Vista 32 to 7 64, you can perform a custom install.
Just go through Setup like you would normally install an OS. Setup will only allow you to pick Custom Install. Instead of formatting your partition, just select the old partition as the target. Setup will warn you blah blah blah. Just click through.
Your old Users, Program Files, and Windows folder will be moved to a folder called Windows.old. And you will have a brand new Windows install to play with.
Note: Setup will overwrite your old bootfiles.