First a correction…. It is not ACHI. It should be AHCI or Advanced Host Controller Interface.
SATA ports can be configured to work as IDE, Raid and AHCI modes. When you setup your motherboard to work on native IDE mode SATA drive will appear to system + OS as a legacy drive. This will not give you the advantages of SATA drive such as Native Command Queuing (simply put, higher speed) and Hot Swapping.
Please read Wiki here… Reading is more voluble than Gold!
Please note that turning SATA IDE to SATA AHCI mode on Windows pre installed system will create booting issues. You need to modify the Registry parameters to fix this issue. Please post back here if you have this condition. I will direct you for a remedy.
You don’t need to enable RAID functions unless you are planning use more than one physical drive to configure as single drive. Raid will give you added speed and or redundancy on a drive failure. This is entirely dependent on how you setup your Raid set.
If you enable AHCI mode in SATA prior to Windows Vista/7 installation, setup will reorganize setting correctly and install drivers accordingly. In Windows XP, you need to provide AHCI drivers in a floppy during installation.