Heres how for a normal 8500 with no pegs
Take the card put it in an anti static baggie and place it in the freezer for 15 minutes as suggested.
Then take a credit card and place it up against the stock heatsink, then place a butterknife on top of the credit card and give it a small twist....The ATI heatsink will pop right off
Then use Arctic Alumina adhesive (especially if your affixing ramsinks) or Arctic Silver Adhesive to affix say a crystal Orb.
Be sure to prep the gpu with Alcohol or Nailpolish remover that contains acetone to get the ATI adhesive off. (if you have steady hands use a razorblade and clean it up with the Alcohol or acetone)
Since you have pegs

Heres a tip, take some plastic tweezers and release the pegs from the backside of the board....the tweazers will help you....Im thinking that they may have epoxied the stock heatsink to boot....If all else fails cut the pins with wire cutters and follow the steps for the normal 8500 Heatsink removal posted above.