What is the most popular raid setup?

Depends on if you want performance or redundancy.

For performance, RAID 0 is more popular. In this setup, data is split and written across the two or more drives that are involved in the setup. It offers the highest performance levels, since data can be read from and written to multiple drives at the same time. It also allows one to integrate multiple physical hard drives into a single drive for storage simplification purposes. However, it's nearly impossible to recover data if one drive crashes for some reason.

For redundancy, RAID 1 is more popular. In this setup, data which is written to one drive is cloned to another drive(s) on the fly, basically giving a one-to-one copy of a drive at any given time. It's important to note that this is not the same as a standard backup -- if you delete data on one drive, it will be gone on the other drive as well. As a result, read performance will be increased (data can be read from two drives at the same time), but write performance will take a slight hit, since the same data has to be written to both drives as well. The advantage is that, if one drive fails, the other drive can immediately replace it, since it basically has all of the data contained on the failed drive. The disadvantage is that the effective storage capacity of a drive system is limited to the smallest drive in the system (if you have 2 1TB disks involved, total storage is only 1TB).

There are others (RAID 0+1, RAID 3,4,5, etc), but most require more specialized hardware. I'll leave implementation up to you, although you might want to place all drives on its own controller for optimum performance (this isn't an issue with SATA and maybe Fibre Channel, but can sometimes with IDE and maybe SCSI interfaces)