Assuming that this is a new install, the easiest way to install any OS is through the SmartStart setup off of CD. It will allow you to flash the BIOS, erase the disks, establish the RAID configuration, and install the OS. Its simple and straightforward.
You have several choices for RAID - 5, 0, 5 with hot swap. I would suggest the hot swap option unless you absolutely need the drive space. It will keep one drive out of the array in case of failure. If a drive fails, the hot swap will take over, preserving (hopefully)your Data. Keep in mind that RAID stripes data across all the drives assigned to it, so one drive essentially is a parity drive. Thus if you use all 4 18 Gb drives in the array and assign one as a hot swap, you will have 36 GB of drive space, not 54.