There should be a BIOS utility for defining the RAID setup. I setup a Dell PowerEdge 1420SC with SATA RAID just last week (5/8/2005). As the server boots you should see something on the screen that mentions the RAID utility. In my case, the controller card was made by Adaptec (2610SA). Like most Adaptec cards, the setup utility can be launched by typing Ctrl+A as the systems boots.
You can then add drives to the array (in my case I added four SATA drives), and choose the type of array (RAID0, RAID1, RAID5). After you setup the RAID at the hardware level you may still need to obtain a driver for your operating system to recognize the array.
For example, I was instaling RedHat 9. I had to download a driver from Adaptec to support their SATA RAID cards on RedHat systems. Prepare a floppy or CD with the downloaded driver (there are instructions for this on the Adaptec web site as well). Once you've prepared your driver disk, boot the system from your distro CD.
Again, using RedHat as my example, the bootable CD #1 will bring you to a boot: prompt. Type "expert" at the prompt and you'll be given an opportunity to install 3rd party drivers. Insert the floppy you created earlier, load the driver, and continue the install. RedHat then goes into the GUI portion of setup and I was able to "see" my disk array. Partition the drive as you like and you're off and running with the rest of the install.
This is not dramatically different than installing Windows NT/2K/XP. Have you ever installed XP from scratch on a new hard drive? During the early blue DOS screens you have the option to type <F6> to install 3rd party SCSI or RAID drivers. Without loading the manufacturer's driver at this early stage, Windows wil not be able to recognize the hard drive. Consequently, Windows setup will fail claiming that no hard drives are present in the system.
Hope that helps.