I'm sorry to hear about your current situation. If I understand correctly, you started with a RAID1 with two 300GB drives and two other disks in the system: a 540GB drive with valuable data and a 350GB drive with essentially nothing on it. You had intended to migrate the RAID1 to a RAID5 by adding the 350GB drive, but unintentionally selected the 540GB drive. Correct? As soon as you did this two things happened. First, some RAID metadata was written to the end of the disk. Most likely there was none of your data that was overwritten by this operation, but this metadata is used by the driver and OROM to identify the drive as a RAID disk. The next thing that occurred was the RAID driver started migrating the data from a RAID1 configuration to the new RAID5 layout. At this point, critical data at the beginning of the disk started getting overwritten. (i.e. the partition table, followed quickly by data at the beginning of the partition on the disk.) If you hit the power switch quickly, then most of your data is probably still on your disk, but without the "index" to know where everything is. Professional data recovery is definitely the safest option right now. There are also third party tools that might help you get data back (or might potentially make things worse). I don't have any personal experience with data recovery tools. In other forums, one tool I heard recommended is called GetDataBack foir NTFS. I can't vouch for it, but it might be one option for you to look into at this point.
Whatever you do, you do not want to boot into Windows right now, as the migration would continue, and more data would be lost. If you're wanting to attempt your own data recovery, you need to make sure the migration does not continue. The first thing you could do is remove the two original RAID1 disks. At that point if you booted, you'd see a failed RAID5 volume with only your 540GB disk in the array. But this would still be problematic because recovery tools need to look at the disk itself, and they'd probably only see the failed RAID5 volume. Given that you ideally don't want to do any other write operations on the disk until you get your data off, if you move the 540GB disk to a system that is not running the Intel storage driver, then that system will see the disk as a normal disk, and not part of an array. At this point you could run whatever 3rd party tools you want to attempt data recovery. If you do not have access to any system that is not running the Intel storage driver, another option (albeit less desirable) would be to mark the disk as non-RAID in the option ROM. This will clear the metadata from the disk so the RAID software will no longer treat it as part of an array. But this is less desirable because in general when you have a disk that you want to perform data recovery on, you really don't want to write anything to the disk until you've gotten as much data off as possible.
As a disclaimer, I am not a date recovery expert-- my advice may or may not be correct, and could make things worse. There are things I mentioned above that could cause easily cause worse damage. The safest option is find and hire a professional you trust to recover your data for you.
I wish you the best of luck in getting your data back.