For many moons, I've been raising the issue of how people get to 64-bit Windows 7, which is a big compatibility shift from 32-bit Windows XP. The obvious answer is via Vista, but is that 32-bit Vista and/or 64-bit Vista? The price includes both, so it's up to you.
Initially, Vista was a bit short of drivers, or else had some bad drivers. That made 32-bit Vista the obvious choice for most people, because 64-bit drivers were even rarer. The same thing applied to software compatibility, only more so, because while it does run 32-bit programs, 64-bit Vista is much less forgiving about rubbish programming.
Now it looks as though the dramatic improvements in Vista drivers etc could be feeding through into the 64-bit version (possibly with a bit of help from the Windows Server market, which is now almost all 64-bit). Anyway, according to Chris Flores on the Vista team's blog.