AHCI mode is "native SATA" mode, and allows the SATA driver to use all functions of the SATA standard, including NCQ and hot swap. If you're installing Windows to an AHCI SATA controller, you have to load a driver using F6 during Windows setup.
IDE or legacy mode is the other option, where the SATA controller emulates a standard IDE controller. In this mode, no driver is required during Windows setup. But the controller will not support NCQ and hot swap on the SATA devices.
You may have a third option, in addition to AHCI and IDE. That would be RAID. If you ever want to add a second drive to RAID-1 your 750 GB drive (which, given the amount of stuff you'll be keeping on that, might not be a bad idea...), and you want to use the hardware / MB RAID feature as opposed to the Windows feature, you need to use the RAID setting. Otherwise, you should use AHCI and use F6.