replace the pulley or the whole assembly
 

As I was delivering pizzas tonight in ole reliable bonne, I heard a rattle around the passenger side of the engine compartment. I think it is the tensioner pulley. The rattle would stop as soon as I put the car in gear and got above 1500 rpms. When I get to the next delivery or stop light and sit still at 1000 rpms the rattle returned every time, without fail. Should I replace the entire assembly or just the pulley? Keep in mind this is my girl forever and I don't want to have to fix something 2X. Is it normal to have just the pulley go out or is it worth it to avoid any future problems and replace the whole assembly?

It could be the crank pulley
 

Are you absolutely certain it is the tensioner pulley? I know that the crank pulley makes the sounds you are describing when it separates. It is a common failure.
With the engine turned off, try to roll the belt along the alternator pulley. If it is the crank pulley, there may be an inch or two of travel back and forth. If you are able to move the belt back and forth a couple of inches, you can look at the crank pulley and see the outside turning with the belt and the inside where the bolt is will stay still. If this is the case, the crank pulley, also known as the harmonic balancer pulley, will need to be replaced. I paid around $95 for one a couple of years ago. It is an easy swap on your car, provided you have access to an impact wrench to spin off the pulley bolt.
I haven't done one of these tensioners yet, can't help you there.

On the other hand, tensioners and tensioner pulleys have both been found to fail occasionaly.

Before we can tell you the best thing to do, you need to get under the hood and find out where your problem is. Go ahead and do the test that Bastard mentioned, but a quick check to do before that is to leave the engine running and watch the belt, pulleys, and tensioner for a little bit.

A bad or loose tensioner will bounce around a bit while the engine is running, you should be able to notice it.

After the engine is off, and you have done the first check for the HB, remove the belt from the idler/tensioner pulley and check each pulley in your system by turning them by hand. You should be able to easily tell if one is bad by feel alone.

U r correct,bastard! Looks like my mech will get another visit! Tanks! :x :x

Mech had it done in about 4 hrs and cost me $140 with parts and labor. He is so awesome! :o :D