Using all of 4GB RAM... questions and Vista versus Linux...

[Linuxguy123]

I have a new HP hdx laptop with a Core Duo T8100 processor and 4 GB of
RAM.

$ uname -a
Linux localhost.localdomain 2.6.26.3-14.fc8 #1 SMP Wed Sep 3 03:40:05
EDT 2008 i686 i686 i386 GNU/Linux

Even though I have 4GB of RAM installed, Linux appears to only be using
3GB of it.

$ free -t
             total       used       free     shared    buffers
cached
Mem:       3106944     777056    2329888          0      60608
419608
-/+ buffers/cache:     296840    2810104
Swap:      2040244          0    2040244
Total:     5147188     777056    4370132

I've read in other posts to this group that the cause of this is that
the BIOS remaps the some (1GB) of memory to serve as address space for
PCI devices, thus creating a memory "hole".   

I understand that some BIOSes allow one to remap those devices elsewhere
in the map.  My BIOS does NOT allow that option.

Questions:

a) On machines that do not allow PCI remapping, is the processor
physically disallowed from accessing that 4GB of RAM ?  Ie have the
address lines from the processor been disconnected from that RAM due to
being connected to the PCI devices ?

b) How do XP and Vista handle this ? Are they limited to 3GB of RAM
too ?

c) I am running the 32 bit version of Linux.  Would it make any
difference to my RAM access if I ran the 64 bit version ?

Thanks