4gb address space limitation for i386

I have an IBM xSeries 350 4xPIII with 5.5gb of RAM, and see that only about 3.5gb is being used under the i386 port. I've been looking through the
archives to try and figure out what the root of the problem is and I amn't
quite sure I know which part of the situation is the real problem.

Is it a limitation of the i386 port? That is to say, if there were a specific PIII (i686?) port, would this problem be overcome? Or is it a hardware limitation? Is it necessary to use a not-clean method to access the extra address space (is this what PAE is?), and there's no clean
way around it, regardless of the port?

I was looking at building a PAE kernel, but was discouraged by the lack of usb and certain SCSI support, both of which I'd really like to have (in the case of SCSI, need to have). Is this lack of support because of an inherent difficulty in the hardware, or could it be overcome in a stable way with modification of the device driver code?

My basic (and very hypothetical) question - if I had unlimited time and knowledge (I have limited both), what direction would I take to get access to all 5.5gb of RAM on this particular computer?

Thank you,
Bob
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