SATA drives: What is the difference b/w IDE mode and AHCI?

AHCI, or Advanced Host Controller Interface, is a more featured way of exposing storage adapters to operating systems. It's fairly new as these things go, Win Vista being the first Windows release to have it out of the box, but is definitely the way of the future. Older operating systems, or those that simply lack the drivers, can't use ACHI-mode interfaces and require older parallel-IDE style interfaces to access storage. This is the reason why you always need to add drivers while installing Windows XP to motherboards with SATA boot-drives.

If you can use it, always use AHCI whenever possible.

Only 2 major differences between AHCI and IDE mode.

AHCI allows for hot swapping of hard drives if the motherboard chipset also supports it, and it also enables NCQ for hard drives if they support it. IDE mode does not allow for either of these functions.

There is a small hard drive performance hit when you use IDE mode due to NCQ being disabled.

Intel PDf's on the AHCI standard if you wish to do some reading

IDE mode allows a SATA drive to emulate an PATA drive for installation of OSes that don't support Sata (notably windows xp). Generally you install, update/install drivers, and switch to AHCI mode. If you're running a more recent OS, it should be a no brainer to switch.