Before you purchase any drives, find out which interface your server uses. Whether it's an IDE based RAID or SCSI. The two types are incompatible, hence the 40-80 pins. IDE drives typically have 40 pins and SCSI have 80. After you have determined which type your server uses, the instructions to set them up are different. With SCSI, you need to set jumpers to identify the drive in the sequence and all this other extra stuff. Otherwise, you should be able to follow what the others have already posted.

Always, when in doubt, check your manuals. Believe it or not, that's what they're there for, they tell you what you need and how to do it.