If your looking for speed, get RAID 0. It splits data into two parts and writes to both hard drives simultaneously, and you'll be able to use the combined storage of your hard drives (Eg if you have 2 80GB hard drives you can use 160GB of space). But if one of your HDDs fail, you lose all of your data.
If you have a lot of important files on your computer, go with RAID 1. It works like RAID 0, writing to both disks at the same time. But instead of splitting the data into two parts it makes another copy and puts it on the spare drive. With RAID 1 though, you can ony use the amount of space of one hard drive (Eg if you have 2 80GB hard drives you can only use 80GB).