I am using Windows Vista Ultimate x64. I had no trouble enabling AHCI mode. (No RAID right now.) As for Windows XP, it comes with all the SATA/AHCI/RAID drivers on floppy and it has all the XP stuff you need in the box. Otherwise it is a really basic bundle, but it gives you what you need.
I am thinking of moving to X64 Vista (business/ultimate), so this is a great news to me. Provided that I can find a decent video editing program.
PS: Did you have to install any drivers manually during the installation?