Quote -
Example:
Hypothese: Two 32-bits softs  and an Os and 4 gb ram installed on motherboard

----On 32 bits system ,like XP ,you can manage 2 and ,with parameters manipulation ,3 gb of ram maximum for loading OS and after the two softs
----On a 64 bits system ,like XP64 or Vista 64 ,you have acces to all the 4 Gb for loading Os and  the 2 soft, each  one can have access to 2 gb max
It is correct ?

My emphasis ^
No. Each program has it's own address space, and will reference as much system RAM as needed.
If you are running two programs (and you are actually running dozens on boot up), neither is limited by anything other than: 'free RAM' + page files.
If a program attempts to use more than available:
a) 'insuffiecient memory' message, aborts command.
b) crashes
Graphics software (and any performance based software) generally just crash; because checking system resources for every single command execution is a HUGE performance drain.
I.E.  If one program (Poser, for example) is using 1GB out of 4GB available; then the second (Vue 6i, for example) can use 3GB without a hitch. Close Poser, and Vue can then use the extra GB.
That would be true for 32-bit or 64-bit OS/applications.

The difference is the size of the 64-bit address space (2^64, or ~17 billion GB) vs. 32-bit (2^32, or 4GB). No current OS can use 16 exabytes of program address space, even if you actually had that much RAM installed. There are imposed limitations. I think 16GB for Windows and MAC, 64GB for Linux, I could be wrong about those figures; but there is some imposed limitation at this point.

Friends don't let friends use booleans.