A documented compatibility problem between Windows Vista and the 790FX chipset's SB600 south bridge component currently complicates running drives in AHCI mode. As Microsoft notes:
When you try to install Windows Vista on a portable computer that uses an ATI SB600 Series chipset, each driver may take a long time to install. Therefore, the Windows Vista installation process may take several hours.
That's if it works at all. One can install Vista by putting the SATA controller into IDE mode, but AHCI mode is necessary to take advantage of Native Command Queuing and SATA device hot-swapping, so it's not a trivial capability to lose.
With a hotfix from Microsoft, we were able to get drives running in AHCI mode on both boards. However, doing so quickly all but requires an auxiliary storage controller. Vista must be installed with the south bridge running in IDE mode. You then have to move the hard drive to a secondary controller, switch the south bridge to AHCI mode, and apply the hotfix and necessary drivers before swapping the drive back. This problem should be resolved by Vista Service Pack 1, but without slipstreaming, that won't necessarily make the installation process any easier