I've just put Windows 7 on a new PC and am mightily impressed. It is the 64 bit version as I needed 8GB of RAM for handling large graphic files and all the other systems, including Linux I believe, are only 32 bit and therefore can't address more than about 3.5GB. It has been very stable and is very quick to start and shut down. Only issue was getting it to join an existing XP based network. It works now but it took a bit of head scratching. Main problem was XP calls the network MSHOME by default and Windows 7 calls it HOMEGROUP and the settings for changing from the default name were well tucked away in Windows 7.
I think that you will find that Linux kernel was 64 bit ready well before Windows and AMD shortly before Intel.
Here is the page
There is a reason why the top 10 fastest super computers on the globe run Linux (not out of the box of course) and memory access is one of them.
This is the significant part:
Linux was the first operating system kernel to run the x86-64 architecture in long mode, starting with the 2.4 version (prior to the physical hardware's availability).[15][16] Linux also provides backward compatibility for running 32-bit executables. This permits programs to be recompiled into long mode while retaining the use of 32-bit programs. Several Linux distributions currently ship with x86-64-native kernels and userlands. Some, such as SUSE, Mandriva and Debian GNU/Linux, package both 32-bit and 64-bit systems on a single DVD-ROM image to allow automatic selection of the best software during installation. Other distributions, such as Fedora, Ubuntu, and Arch Linux, are available in one version compiled for a 32-bit architecture and another compiled for a 64-bit architecture.
There is an attempt to run kernel in compatibility mode and to be able to run 64 bit applications in a 32 bit kernel. Name of this project is LINUX PAE64 [2].
64-bit Linux allows up to 128 TB (140,737,488,355,328 bytes) of address space for individual processes, and can address approximately 64 TB (70,368,744,177,664 bytes) of physical memory, subject to processor and system limitations.