ICH8 does support AHCI, Intel just doesn't want you to know. If you update to the latest Gigabyte BIOS, there's a new option in there to turn on AHCI.
However, getting the drivers to work properly is an issue. Intel doesn't have ICH8 drivers that support AHCI (no idea why), but there are FAQs out there to hack the inf files to have AHCI turned on (Google it).
Here's a quote from techreport (http://www.techreport.com/reviews/20...5/index.x?pg=2
"Update &mdash Some of Intel's datasheets incorrectly indicate that the ICH8 lacks support for AHCI, but Intel tells us that's not the case. The ICH8 apparently does support AHCI, and thus NCQ as well, but an AHCI driver is required. That driver is supposedly built into Windows Vista, but we're awaiting confirmation on whether an equivalent exists for Windows XP. In either case, our ICH8-equipped P5B-VM motherboard lacks the necessary BIOS switch to enable AHCI"