According to Brandon LeBlanc almost half of the Windows 7 has been installed from x64 image. I think this is not due to the fact Windows 7 does something special with this architecture (well, comparing to Windows 2000 it does: it works on it =) ), but more to the fact that OEM vendors install x64 and there is very many computers with more than 4GB of RAM. Anyway, Brandon says “Happy 64-bit computing!” and couldn’t agree more.

Source: http://windowsteamblog.com/windows/b/bloggingwindows/archive/2010/07/08/64-bit-momentum-surges-with-windows-7.aspx