What HDD? If it is a newer drive use AHCI. AHCI is a subset of RAID bios, just not with actually raiding drives. IDE is the older legacy mode, but will work, although at least with SSDs reduces performance.
If you already have your OS installed, you must first change the Windows registry key to use the msahci.sys driver before you change the bios or you will get a BSOD.
You should use RAID only if you are actually setting up a RAID configuration with several HDD's. Even then, you don't have a RAID array until it is created using the built-in RAID management software.
The IDE Emulation setting is only for use when you are using Win XP or some earlier version. Those OS's do not have built-in drivers to understand AHCI devices, so the "proper" way to use them is to install special drivers when the OS is first installed. But the easy way is to set the BIOS Port Mode to IDE Emulation. This has the BIOS intervene and make the actual SATA HDD appear to the OS as a simpler IDE unit it does understand, and it works. However, you do lose some of the properties and performance of a true AHCI device.
If you are using Win Vista, Win 7, or anything after Win XP, you should always set to AHCI, because those newer OS's DO know how to use the AHCI devices properly.