I have a Gigabyte GA-990XA-UD3 with one HDD connected to SATA Port #0. Can I install RAID 0 on #4 and #5 without totally starting over? To enable RAID for these, I set the On-chip SATA type to RAID and On-chip SATA port 4 and 5 to "SATA type". I have two 250 GB HDDs sitting in the bays I would like to run games on, but would like the OS on the single HDD. I was not thinking about this when I installed the OS, so I'm stuck in IDE. From what I have read it, seems like a nightmare but everything is until you've done it. My CPU is an AMD 990x, southbridge AMD 950.
I used Acronis TrueImage Home to do something similar. I had Windows installed on a 500GB drive in IDE mode, and wanted to set up RAID0 with a second one. To do this, I imaged the drive completely with TrueImage and created a bootable recovery disc, downloaded the RAID drivers from my motherboard's website and saved it to an external USB drive, and then with the TrueImage recovery disc, used "Universal Restore" to restore the disc image backup I just created. During the process, you are prompted to point it to the RAID drivers, which allows it to install them into the Windows image while it restores it. If you don't do that, Windows won't be able to boot. But if all goes well, after the recovery process completes, Windows should boot and everything should run just fine, as if nothing happened.
But just keep in mind that since you'll be using RAID0, it will double the chances of a hard drive failure killing your computer (since your data is spread across two drives instead of just on one), so be sure to make regular backups, just in case.
NOTE: I am in no way affiliated with TrueImage or Acronis. There may be other software out there that can do the same thing; I just had good luck with this particular one.