The main advantage of RAID READY is that if in the future you intend to add two or more sata drives to the system and you want them to be configure in raid mode.
However, if you have a system where the controller is in AHCI mode, then you can never switch to raid, you will need to start all again from scratch. Like reinstalling the windows, drivers, etc.. (I have seen on different forum that there is a way to do the switchover without reinstalling an OS, but i have never tried.)
If a system is configured in raid mode using the intel matrix controller, the same benefits that you get from ahci, will be applicable as in the raid mode.
Now if you intend to setup the system as raid ready, make sure to install the f6 driver for windows xp. Note: Win Vista, 7/208 have this driver in its kernel, so you wont need it.
All the best,