A Google search for "IDE vs AHCI" would answer all your questions so I'll give you the short answer. AHCI mode allows you to utilize some of the technologies introduced with SATA drives like native command queuing (NCQ) and hot-plugging. Hot-plugging allows you to remove and replace a hard drive while the system is running. I'll let you do some Google searches on NCQ since it's a little more than I want to put in this response. To start you off, I'll give you a link found by doing the Google search I recommended.
I would use AHCI mode for a fresh Windows 7 install. I don't see any drawbacks to using AHCI mode.
I have the GA-EX58-UD4P MB also. Rebuild with OCZ 128G SSD. BIOS would not let me install Win7 using AHCI mode (for TRIM support).
I install in the IDE mode and try to boot with AHCI, system hang during MB posting.
I have firmware F13, the latest. I thought the intel chipsets is already preloaded to allow AHCI Win 7 install. Am I missing something? Help!!!