As far as I am concerned a computer with an OS with 32bit can only use about 3 to 3,5 GB of RAM.
Depends on the BIOS. You can have and use 4Gb of RAM (I have it on Vista).
I don't think so

Video memory, and other mapped stuff, eats the 4GB address space.
Actually, it's kind of a artificial limit - in XP SP1, it was possible to have&use more than 4GB RAM (OK, not for a single application, but in general), but since XP SP2, it's not available anymore.