The point, mainly, is that a 64 bit OS supports more than 4 gigabytes of virtual memory whereas a 32 bit
OS supports only 4 gigabytes of virtual memory. Since in 32 bit all of one's RAM plus the RAM used for
one's PCI/PCIE/cardbus/etc. peripherals as well as one's swap virtual memory must all fit within 4GBy,
one will often not be able to use more than 3 GB system ram plus a video card without running out of
address space.
When I install a 2nd video card on my main system which has 3GB ram installed I cannot use all my RAM
and a few hundred megabytes of it becomes useless in 32 bit OS environments.
Also the added virtual address space of the VISTA 64 bit OS allows it to somewhat "randomize" the place
in virtual memory where parts of the operating system, your programs, your DLLs, etc. are loaded.
This randomization helps the OS be more secure against malware since often in would be necessary
to know the address of certain parts of the programs and operating system data in order for a program
to attack vulnerable spots of those programs / data structures at their predictably known addresses.
If the malware can't rely on the weak points of the system being at predictable addresses in memory
it makes the system more resistant to its attack since the programs and data are loaded more randomly
around virtual memory in 64 bit.