I have send a query to some of the BIOS folks who worked on this board (hope they remember that far back ). Being Saturday, it will likely take a few days before I get a response...
Ok, after some research, I was able to verify that this board, a 4-Series board that utilizes an ICH10, definitely has support for AHCI (in fact, the ICH9 that was used on the 3-Series boards had AHCI support). Why the BIOS doesn't offer an AHCI option is a mystery. This morning, I talked to a couple of the BIOS engineers who were in the group back then. Their albeit faint recollections (we *are* talking 9 years since this board's BIOS was developed) were that AHCI was supported in the BIOSs at that time but it wasn't as yet being set as the default mode of operation. That it wasn't offering AHCI as an option in your BIOS was a mystery to them. They also indicated that they believe that RAID support is dependent upon AHCI support, so enabling RAID should enable AHCI. You definitely want AHCI enabled. AHCI utilizes memory-mapped I/O. That is, it uses memory space cycles, not I/O space cycles as Legacy mode does. I/O space cycles serialize processor operations and slows everything down (everything else halts until the I/O operation is completed). In addition to improved disk performance, using AHCI means that disk accesses can occur in parallel with other activity, improving overall processor performance. Having to enable RAID to enable AHCI is not a penalty; until you actually create a RAID volume, it is just performing AHCI operations. You do not absolutely need to install the Intel Rapid Storage (RAID) software; Microsoft provides an AHCI driver as part of its base Windows driver set.
Hope this helps,