Shuttle IOMMU question on x86_64 system?

Hi,

A question for the group here. Recently, when checking out the
"dmesg" and "messages" files on the current rig I have (a Shuttle
SN27P2 barebones system with a AMD Athlon 4600+ dual core CPU in it
running both Mandriva Discovery and Kubuntu "Dapper Drake" 64 bit OS),
I noticed there was a line listed in both files that read:

CPU 0: aperture @ f39c000000 size 32 MB
Aperture from northbridge cpu 0 too small (32 MB)
No AGP bridge found
Your BIOS doesn't leave a aperture memory hole
Please enable the IOMMU option in the BIOS setup
This costs you 64 MB of RAM

Now my quick question for the group here is, is this something I
should be concerned about? A check of the BIOS chip doesn't show any
IOMMU setting as far as I can see (I assume this setting would be in
the BIOS?). There is a line farther down in each of the above
mentioned files that references something called a GART IOMMU freeing
up some of the memory (I think?). I'd really appreciate any help
anyone can give me resolving this, with me basically being a Linux
newbee. Thanks for any assistance anyone can offer to me!