oblivious_maximus, have you already tried removing all the mtrr and other changes and just limiting memory usage to 3.5GB or so with the "mem=" boot command ? I don't know the details but apparently this has worked for a few people here. Having the SBIOS properly remap RAM is best but that doesn't seem to be available to everyone on the current generation of motherboards ;(
EDIT - just found your other post where you asked for details on how to use the mem= line, and of course now I can't find any of the original posts by other people who had success with it. Auggh.
Anyways, I would start with something simple like "mem=3000M" and see how that works.