One of the main reasons for the shift: Vista 64-bit can address more than 4GB of memory.
|According to a post by Chris Flores on the Windows Vista Team Blog, the proportion of all US machines with Vista running the 64-bit edition has tripled in the past three months while worldwide adoption has more than doubled in the same time frame. Microsoft’s claims are based on download figures from Windows Update, though the company isn’t giving any hard numbers.
Among the factors leading to the shift appear to be the fact that 64-bit Vista machines, unlike 32-bit systems, can directly address more than 4GB of memory – an amount rarely needed in 2005 when Microsoft released Windows XP in 64-bit form. Also, 64-bit drivers are required for WHQL certification in Vista so hardware compatibility has become less of a problem nowadays.