As you have already gathered, the D80 is not particularly well suited for IR photography.  The ccd filters and even the newer coatings on some of the lenses inhibit the ability for good IR photography.
Saying all of this, the D80 can produce IR photos...to test for yourself before you spend $150 on a R72 filter....take a TV remote control....press the button...take a picture of the IR emitter ( while pressing the TV remote button) with your cam.  If you can see the distinct light being emitted by the TV remote, your cam will be able to take IR images.  On some cams. the remote light will be very bright and distinct...that means it is a good IR cam....on some the light will be quite dim...in other words not well suited for IR work.
You will find the D80 to be not as well suited as say the Coolpix 5000.

If you do want to do a conversion of your cam...there is a company that can do this for you....I think it is called "lifepixel".

In youth, we learn....with age, we understand.