My Dell has IDE or RAID options only, I changed it to RAID and never had any HDDs setup in RAID, they are seen as non-RAID drives.
It will work with one HDD or several, none have to be in a RAID array.
If your BIOS is set to RAID you will have all the benefits of AHCI, so no need to change it to AHCI.
If you still want to change it follow the link provided above by theog.