"...the simple act of switching to RAID mode meant the already-configured drive with Vista on it wouldn’t boot."
I tried this on the 780G board. I had xp installed on my SATA drive but everything was in IDE mode and I wanted to switch to AHCI mode. When I changed the option in BIOS, it gave me another option to let SATA ports 4 and 5 remain in IDE mode while the other ports were switched to AHCI. I connected my drive to port 4 and windows booted. After loading up, the "Found new hardware" wizard started and it installed the AHCI drivers from the motherboard CD. I then shut the computer off, switched my drive to port 1 and Windows booted. HOWEVER, with AHCI enabled, the boot sequence would pause for about 10 seconds which became really annoying (XP loading bar would freeze) and I didn't feel like dealing with it so I went back to having everything as IDE. Maybe you'll have better luck.