It's because you are out of MTRRs. It has to do with memory holes and BIOS (I have not looked at the Wikipedia page so forgive me if that's what you already know)
I've never really seen much on "fixing" it (seems that it's more of an issue on laptops and devices that share RAM with a GPU) but you might try messing with the AGP "memory hole" option if it exists in the BIOS. I assume your BIOS is the latest-n-greatest.
This may offer some insight into altering the MTRR use in the OS:
http://www.mplayerhq.hu/DOCS/HTML/en/mtrr.html....but there's likely LOTS of options not mentioned there (case in point, I think write-back is the more common config type)
Don