Thanks for the responses. I'd rather not do a clean install, because I have lots of software installed on my Vista OS - and for some of it I don't have the original setup files anyway so I couldn't reinstall it.
You can upgrade to any version of major versions of Windows 7 64bit.
So I don't need W7 Ultimate - I can use Premium or Professional?
I want to make sure because the Windows 7 Upgrade Advisor software says:
Upgrade optional available
You can upgrade to 64-bit Windows 7 Ultimate.
I am not sure if this means I can *only* upgrade using W7 Ultimate, or if it is simply listing one option out of multiple.