Wow, I can't believe how many posts there are on this thread that are so far off topic. They answered every other question, except the one you asked.
Only coldsleep commented on the topic and I agree with everything that he (or she) said. Coldsleep seems to have some valuable experience.
#1 Put the OS and programs on a RAID-0 -- using two drives
Put two more drives with the data on RAID-1 -- redundancy
#2 put OS/programs and data on 3 or 4 drives using Raid 5
#3 Put os/programs and dates on 4 drives using Raid 10
I would probably go with a single drive for boot, and a RAID 10 with the other 4 drives. If not that, then option 3, 1, and then 2. Good luck and have fun.
Other thoughts related to this topic:
I have had never had a drive fail, but several of my roommates had. One of my roomies even had his main storage drive fail, and then his external backup storage drive failed 30 hours later. He lost everything. He was devastated.
Those drives were both about 1 year old, and were different models and capacities. Drives do fail. No matter the brand, either. Don't ever try to say that they don't. To say that they don't fail is ridiculous. They may fail rarely, yes. But when (not if) it happens to you, it will undoubtedly be too soon, am I right?
Regular backups are important too, just in case you delete a file that you meant to keep.
But backups can only save data from a particular moment in time (say goodbye to that file you created 2 minutes ago), and are timely to create and to restore data from. That's where RAID arrays come in. Instant redundancy is not foolish or overly-cautious. It's smart. Expensive? Depends on the value you place on your files I guess.
After my roommate's hard drive's failed, I implemented a RAID 1 for my storage using using an integrated motherboard controller and a pair of 250 GB PATA drives that I had purchased refurbished from Woot and had previously been using in different capacities. As it was my first time with such an experiment, and none of my friends had ever implemented a RAID, I was a nervous noob. But it worked fine.
When it was time for my next build, I started in a similar position to where beenthere22 is now. In 2008, I purchased four 500GB HDDs and did some testing. I tried a RAID 1 with two 500 GB drives split into 2 partitions (2 bootable OSes, WinXP32 and Vista64, and each with accompanying software). I wanted to be up and running no matter what. I also used another RAID 1 with the remaining 2 drives for my main data storage, such as documents, photos, music, and movies.
For the most part it worked great. But I did learn a few things through a long series of frustrating bumps along the way. All in all, I am pleased with the knowledge and experience I've gained, and above all, the peace of mind that my data is pretty darn safe.
On month after completing my new 2008 build, I took my new system over to a friend's house for a LAN party. I got there and started up my system. The system turned on but would not boot successfully. Finally I could see that I had a failing OS boot drive. No matter! I simply opened up the machine and unplugged the data and power cables connecting to the bad drive. Total down time: 12 minutes. I was playing games with my buddies as if nothing had happened!
Since then I have moved away from using a RAID on my OS drives, only due to lack of funding for more bigger drives.
Lessons I've unfortunately had to learn:
1) Sometimes a bad SATA cable can be the cause of all your problems. Or it could be just the connector on one end of your cable. Or it could be the data connector on the HDD or on the motherboard is bad. Be extremely careful when detaching SATA cables--you should take care to press the release latch every time so as to avoid damaging your expensive HDDs. After months of troubleshooting various components, I finally replaced all the SATA cables that were included with my motherboard and low and behold I finally stopped having RAID sync errors. Imagine that.
2) You should be able to reboot your machine during a disk re-sync with no problem. I mention this because a re-sync can take a long time, 1-3 hours usually. However you may want to check your documentation first.
2) Because the motherboard I have been using is an EVGA NVidia 780i, if I want, I can install the Motherboard drivers and it allows me to control all the RAID settings via the NVidia control panel. The downside is that there is no way to gauge the temperature of any HDDs that are in a RAID--at least that's how it is with this motherboard.