Fedora 9 32 or 64 Bit - Which One?

[stan]

Alan Cox wrote:
> On Mon, 27 Oct 2008 11:54:47 -0400
> Matthew Flaschen <matthew.flaschen at gatech.edu> wrote:
> 
>> Alan Cox wrote:
>>
>>> The moment you have more than about 900MB of RAM there are big advantages
>>> to running a 64bit kernel as it can keep all of physical and virtual
>>> space mapped at the same time, which is a big performance win.
>>>
>>> Alan

Wouldn't you need twice as much memory to have the same 
memory for applications if you are using double the word size?

Or does the OS somehow take that into account and split the 
64 bit words into their components to get most efficient use 
out of them?

To clarify, I have 2 GBytes of memory in a 32 bit OS.  If I 
use a 64 bit OS, isn't that memory now effectively halved? 
The same as if I use 16 bits to store a character instead of 
8 bits.  It is my understanding that UTF-8 only uses the 
second 8 bits if it needs it.  So that is like my second 
question, making sure that there isn't lots of empty memory.

Thanks for any clarification you can offer.