W2K AS install – disk setup?

I am setting up an Exchange 2000 server, and I’m not sure what the best way is to allocate the disks. It’s a Compaq ML370, 1G RAM, dual processor, 431 RAID card, and 5 18.2G Ultra3 10K HDDs. We want to maximize the amount of space.
How should I set the drives? I have been told to mirror 2 drives for the OS & swap file and set the other 3 in a RAID5. To me, using 2 drives in a mirror seems excessive for just the OS. Does anyone have any good suggestions? I can purchase an additional 18.2G drive if necessary.