You'll know when your throwout bearing is toast. It's a sound...that sounds very, very, bad. Anyways, it's always a good idea to get the clutch kit as a whole- it saves you another trip later down the road for a new clutch. Shop around and get some install estimates- it can range anywhere from $250-500 for a clutch install. Keep in mind that you'll need to either have your flywheel resurfaced or buy a new one, and that the clutch fork and ball thingy (I can't remember what it's called) may need to be replaced.
All told, it cost me a little over $700 in parts and labor about two months ago for mine.