Running a SATA drive as AHCI allows you to hot swap the drive, if you have a hot swap bay in your case. It also allows drives to use Native Command Queuing if the drive is equipped with it. Some say there is a performance increase to AHCI and NCQ, but when I converted my drives, I really didn't see any boost from it. YMMV.
One thing about SATA drives. The older SATA1 drives are slower than the newer SATA2 drives, and the SATA3's that are coming out now. It's just the interface between the disk and the motherboard. There's no problem mixing them, but you may be confused if you do, and wonder why one drive is so much slower than the others. Most SATA2 drives have a jumper setting on them that runs them at half transfer speed (SATA1) for compatibility with older motherboards. It's the first thing to check if you think your drive is too slow and you know for a fact it's a SATA2. I've seen the jumper come that way from the factory on occasion, which is really weird.