FAQ - Does 64-Bit Windows Help Memory?
The short answer is yes. The 32-bit versions of Windows XP and Windows Vista can't "see" an entire 4GB of RAM. Windows XP (and Vista) can address up to 4GB of physical memory, but some of this space is reserved for hardware drivers and I/O addresses. Under a 64-bit OS, Windows has access to a much larger physical address space - up to 128GB of physical memory - so this address-space quirk is not an issue.