RAID 10 which is really RAID1 + RAID0 combined has the highest fault tolerance (you can lose up to 2 disks in a 4 disk array) and fast rebuild times and great all around read and write performance if used with a good controller. With the RAID10 you have two RAID1 arrays striped with RAID0, this effectively increases disk transaction rate beyond what single HD can do by itself. Combine this with a 3Ware or Adaptec controller that is capable of NCQ and has dedicated CPU and 512MB or more of onboard RAM for data caching and we are talking about serious power. The drawback on RAID 10 is the wasted disk capacity. For example to configure RAID 10 you need four disks, the final array capacity will equal half of the sum of all disks or (n*4)/2. Considering the availability of >TB disks and low costs, there is absolutely no reason why you should use RAID5

For most implementations this would be an overkill in my opinion. We commercially host DNN sites, and we do truly use every single IO the server can put out on certain occasions. When things get busy, you will want your server be able to keep up with the disk IO pressure. We have had customer sites that went viral without prior notice and these configurations can sustain loads with over 3 million hits per day.