I can only agree with what Christian says.
One question: what computer are you using?
If, for example, you have a MacPro, it has four SATA II drive bays - - so all you would have to do is to purchase a bare drive, attach it to one of the drive trays that come with the computer and slide it into a drive bay. If you want to use an external eSATA setup then you will need a card, a drive and drive housing. Again this will be either PCI-e or PCI-x depending on your motherboard. (MacPros need a PCI-e card). Among cards, you can choose those which allow you to have software RAID levels 0 and 1 only or those (more expensive) cards which enable you to create other RAID levels such as level 5 and 6 which combine speed and redundancy. (For more about RAID levels, check out Wikipedia on this subject.) If you have a Mac you might want to check out the exhaustive reviews of cards, drives and drive housings available at the Arizona Mac Users Group website (www.amug.com) - - if you have a Windows machine, these reviews would still be helpful since all the drives and housings and almost all the cards work with either OSX or Windows.
One point: there are only a few manufacturers of hard drives, such as Seagate, Western Digital, Samsung and Hitachi. Glyph, for example, makes drive enclosures and bridge circuits necessary for Firewire and/or USB, not drives - - it uses drives made by one of the hard drive manufacturers.
One of the advantages of eSATA external drives is that they do not need bridge circuits - - all the circuitry is on the card. The only major disadvatage of eSATA drives - - at least in regard to the MacPro - - is that, at the present time, as far as I know, only two cards, the Areca 1221x ($720 at newegg.com) and the HighPoint RocketRaid 3522 ($460 at Other World Computing) allow you to boot from a drive or array connected to the card. Another card offering bootability, made by CalDigit will, according its manufacturer, be released in June. According to CalDigit, this card will work with both OSX and Windows and also with BootCamp on a MacPro.