Depends what you want to do.
If you want better transfer rates then RAID0 will give you that although if you want to boot from the array it's reinstall time.
If you do stuff which has a lot of simultaneous I/O then you might get better real world performance by running the disks individually. For example when encoding video read the source file from one and write the output to another.
RAID1 has been mentioned but remember RAID1 is NOT the same as a backup. It provides continuous availability in the event of a hardware failure, that's all. If you (or something else) delete or corrupt a file then it's gone, same as on a single disk.