These are just some thoughts here - I don't know the real answer to your question per se.First and foremost, the most common two RAID options (for PC's with typical setups) have some properties that make them useful:
- RAID 0 gives you speed. Every other chunk of data (selectable between 2k to 64k for most RAID setups) is read from, or written to, the "other" disks in the RAID array of disks. This is usually a major speed enhancer, since physical drive read/write rates are slower than the bus rates. Thus, the bus runs at high-speeds and the RAID divvies up the reads and writes to multiple physical disks. In most PC configurations, 2 disks are sufficient to optimize the speed as good as possible.
- RAID 1 gives you reliability. Both drives are written in parallel, although reads are alternated. So, if one drive fails (hardware wise), then you simply pull it and use the other totally normally. Till you replace the broken and re-sync the old to the new. Again, two disks are usually the right number for a RAID 1 array, unless you want more reliability. In either RAID 0 or 1, if you plan to use more than two drives in an array, then selecting some other RAID option is usually better.
Thus, there are other RAID setups, usually requiring more drives, that provide additional speed and/or reliability or combinations. More than I can elaborate on here ... do a google search for info.
Note: With RAID 0, you get the capacity of the two drives added together (so, two 500MB drives in RAID 0 give your a capacity of 1TB) and with RAID 1, you get the capacity of the smaller drive in the array (so, two 500MB drives give you a capacity of 500MB).
BTW, it is best to operate with identical drives (sometimes even to the point of having the same drive firmware) in the RAID array! 
Now, on to the original post. A few things come to mind:
1. Usually, you have to enable RAID functionality in the BIOS of the motherboard. After you do this, some other screen sequence pops up during the boot that allows you to use the RAID firmware to actually set up the array of drives. I.e., to configure RAID the array, select RAID type (0 or 1 or ...), format the array, etc., etc., etc.
In my Alienware MJ-12 laptop, the Promise Technology RAID chipset is enabled first in the BIOS, and then during boot, I get a prompt to press "Control-F", or some such sequence, to go into the RAID setup screen.
If you never see such a prompt, then your BIOS is not selecting RAID instead of SATA.
BTW, this is NOT what you are experiencing, I believe, since you do see the sequence during boot and the controller shows a proper RAID array.
2. The second thing that happens is that the drives are no longer visible as IDE or SATA drives - the RAID bios must show them to the OS as some other kind of special storage drive/device, otherwise, the OS would install/use the wrong driver. Typically, this means that you also have to have a driver for that RAID setup for the OS of choice.
This means that, during the install process, you will not see any standard drives unless you load a driver (this is an Advanced option during the drive select stage for the OS install).
In my laptop, the Promise Technology RAID chipset shows the drives as a special SCSI drive, and they provide a driver for it. My one problem was that the driver I have is old ... was intended for Windows XP, and they have not updated it for other OS's. 
Fortunately, this old driver has worked for me, both for Vista as well as Windows 7. So, I recently upgraded to Windows 7 Professional and was able to use the old driver (read it from a USB stick) during the OS install process. Whew.
Anyway, this is what I suspect is your problem. Use the ASUS software disks (from the floppies) and use them to select whatever XP or Vista driver you have for the RAID chipset on that motherboard. During the OS install process.
Hope this helps ...
Z
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