If it were me, because I'm not an expert on exactly what *might* happen, I would tar.gz the /usr folder and back it up in case anything goes totally haywire and you need to restore it for some reason (unless you have a CD or DVD to recover from).
Then I'd just go ahead and install.
Myself, I have not been following the details in all of the READMEs and Upgrade.txt files, because I chose to install -current fresh. You really should read ALL of such files that have been made available, before deciding exactly what to do.
Your /home folder will be fine, with regards to personal files & data, music, pictures and everything like that.
However, as far as the /home folder: depending on how much difference there is for example, in your .configuration directories/files for like KDE (for example), you never know how the upgraded desktop environment and other upgraded applications will react when you log in and they discover the OLD configuration files. If there's enough non-compatible stuff in the old configs vs the new configs, the applications or desktop might act weird. You ought to investigate whether (for example) KDE or whatever can upgrade cleanly from your current version, to the new version. I would check the KDE (for example) website, or wait until someone else who has done this can tell us one way or the other.
On the other hand, the new KDE (for example) might just overwrite the old config with a new default, and you reconfigure fresh.
One tip: (EDIT
: This tip may be irrelevant; see post #4)
I don't to my knowledge use any 32bit stuff either, at least to this point, but I have not installed anything from outside the 64-current tree yet either. I'm running 64-current's packages, and that's it for now, to test stability. BUT: (here's the tip) If you happen to use the nVidia binary video driver, (this is what happened for me) you will need to select/install "Nvidia's 32 bit compatibility libs" when running the driver installer, or X won't work.
I actually don't know why this is, as I have not yet looked into it; I'm sure the explanation is readily available somewhere. So my point: there will be a 32bit /lib directory created or required ANYWAY, if you use that driver. So if you leave /usr as is, and upgrade over it, chances are, everything will work; BUT -- you may end up with some extra leftovers in there.
I know this doesn't fully & concretely answer all your questions, but I hope it gives you a little bit of an idea what to expect.
Perhaps Robby or Eric will pop in and clarify, and/or correct anything I've written & enlighten us both