With that motherboard, you still can run RAID on the ICH9R chipset, even if you already have your other drives running. Just in case the Marvell controller flake out.
I don"t like either JMicron or Marvell controllers. I completely trust Intels. Isn't the Marvell controller for that mobo for SATA III? That's okay, SATA II drives will still work on a SATA III port, but you will not get any benefit in speed. SATA III is overrated for hard disk drives (HDD), it's really meant for Solid State Drives (SSD) coming out.
Anyway, if you already have the OS and additional drive sete up, you can add more drives. If you use RAID on the Marvell controller, the mobo CD/DVD should have the drivers on it. Check your manual.
You can set up RAID in the BIOS for which ever controller you use, and it SHOULD have no effect on the current setup. I said should. If you use Marvell, you would only set up that controller in BIOS, and then enter the RAID set up during post. Something like <Ctrl>+<I> during post enter Intels RAID setup, so Marvell should be close. Check your manual.
If you wanted to use the Intel controllers, you still would set the SATA to RAID in BIOS, and enter RAID setup during post. You can set up a single drive in RAID, and it'll still be a single drive in Windows. Should be no changes. It'll use ACHI as default. You would add the 2 new drives, and set them up as RAID in the same set up. Upon Windows boot, you will need to allocate/partition/format the new drive array via Disk Management.
If you need more detailed info, let me know.