Replace my Hard Drive

No, you don't.

But! you do need to use a special adapter cable as Apple leverages the HD's internal thermal sensor to manage the systems cooling. Apple contracted special drives which are only available from Apple (which are very expensive!)

Using this adapter cable: OWC - In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade allows you to put in any HD or if you want you could even put in a SSD instead. Frankly, I would recommend you do the dual drive config if you want a SSD.

If you are planning on just replacing the HD with another HD I would make the jump to a hybrid drive like this one: Seagate Desktop SSHD