It really depends on the software. I'd like to think a lot of programs will work, but not
every developer of music software is jumping on the 64-bit bandwagon just yet. Odds are
you'll find programs are more compatible with Vista then with XP-64 bit (as was my
experience), and if you run into trouble there's more chance of the company that made the
software being willing to help, but you could still run into issues you just aren't able
to solve in some cases.
For what it's worth, I have EWQL symphonic orchestra
silver (the non-play version) and it runs just fine on Vista 64, which is more than I can
say it did on 64-bit XP where even trying to open it immediately crashed my computer.
Never could get it working on XP. All of my other plug-ins I've installed have worked fine
as well. Other people may have differing experiences however.