I recommend fresh-installs for major updates like this, but if you want to upgrade, the (ridiculously complex) upgrade matrix is available from Microsoft here: http://www.microsoft.com/windows/windows-vista/discover/blog.aspx?p=c3d7d289-3ae2-43ea-b275-2c7773d0f9a2
Here's a summary:
Upgrades can only happen from Vista to
7, and only when upgrading to the same
architecture (ie, 32-bit to 32-bit or
64-bit to 64-bit).
If you have Vista Home Basic or Home
Premium, you can upgrade to 7 Home or
7 Ultimate. If you have Vista
Business, you can upgrade to 7
Professional or 7 Ultimate. If you
have Vista Ultimate, you can upgrade
to 7 Ultimate.
Any other combination (switching 32-bit to 64-bit, upgrading from/to a combination not listed above, or upgrading from XP or prior) becomes a "Custom install" (ie, install from scratch).