Some ssd's don't support AHCI officially and could become unstable when switched from IDE. In any case, if you are trying to switch and your system is installed on an ssd, you may need to make some registry changes before you change it in the bios. I found the following guide from OCZ: http://www.ocztechnologyforum.com/forum ... stallation
I think you need to edit the registry only if you already have windows installed on an ssd and you are trying to switch to AHCI. If you are just adding a (non-system) ssd drive or making a fresh windows installation (after already changing the bios) you probably don't need to make any changes in the registry.
You don't need to mess with registry. Make sure you have have ahci turned on in the bios. When you install windows will automaticaly configure for you. I set up my SSD card today and no problem so far.