The only version of Vista that
arrives with both 32-bit and 64-bit versions in the package, is Vista Ultimate in the
retail-boxed version. If you buy a
retail-boxed version of Home Basic, Home Premium or Business, then you get just the 32-bit disc in the package, but the 64-bit disc can be ordered
from here if you'd prefer 64-bit.
If you buy OEM, then you commit to 32-bit or 64-bit at the time of purchase. What you buy is what you can use.
Either 32-bit or 64-bit Vista will be more secure out of the box than WinXP, and make it more practical to routinely use a
non-Admin user account for resistance to exploits, viruses and spyware, if you're interested in better security at the expense of the ease of use of of-course-I'm-an-Admin-what-else-would-I-be?-

operation.
The 64-bit version of Vista has additional security features (you can read an overview
here, scroll down a bit to
64-bit Security Features). It can handle more RAM. Its compatibility with old software and possibly drivers for old hardware could be issues.
In any case, if you want to see if your software will be OK on Vista, run the Vista
Upgrade Advisor and see what it reports. It notified me that my old image-editing software was going to have Issues (which it does) and that my antivirus wouldn't work (but as it turns out, there's an update from the antivirus company that fixes that).