Yep, that too. People think the limitation is 4gb of RAM, but it's 4gb of addressable space. That video card RAM, NIC buffer, and tons of other things need to be in the addressable space of 32 bits somewhere, so it pushes out RAM from being usable. I didn't even try to load a 32 bit os on my Mac Pro, but a friend did on his, and he only gets 2gb usable.

Certain higher end servers running 32 bit OSs would boot with a gig or less of usable RAM, due to all the address space reservation for hot plug PCI slots and other components. That was where PAE was used as a bandaid until things could move to 64 bit on x86. PAE doesn't work well for performance though, and it's pretty much a nogo for the consumer space since the video card RAM often gets pushed out into a separate 4gb space that has to be paged into.

Intel and Microsoft were so convinced Itanium was going to be the 64 bit migration path that they didn't bother with a plan B. And now many of us get to feel the fallout of that decision years later. It's a shame AMD is falling behind again, their Athlon 64/Opteron was a good kick in the teeth to Intel to get them back in gear. Sure it cemented x86 as the common PC architecture for a while longer, but it prevented everything from hitting a brick wall when the Itanic sank.