The adress space limit of 32bit is 4GB, the maximum memory space the OS can adress. Adress space includes not only RAM but other memory like graphics memory (the graphics card memory), so if your video memory is 512MB that will eat up 0.5 GB off the 4GB adress space limit, other devices also need to reserve certain amounts of adress space. What all this means is that, on 32bit OS the maximum RAM you can use is in fact about 3GB, but it can be more, depends very much on the graphics card on-board memory.
