Hrmmm. I have a dell optiplex 745 at work, but it's not sff. I have no problem using the drives in AHCI mode. In fact, it came set that way.
Also, depending on your OS, you may need to install the AHCI/SATA drivers. If you have vista or 7 already installed, you need to make some registry changes before switching the bios over to AHCI.