be sure to put an old credit card between graphic card and screwdriver. otherwise the twisting will scratch some new traces in your graphic card, and you certainly don't want that, although the card would then be really silent, DEAD silent
i have recently popped off a glued on heatsink from a radeon 9000 aiw. no freezing or heating, just credit card - screwdriver - twist. it made some cracking sound, then another twist and it was off.
twist firmly but slowly. you don't want to scratch that screwdriver straight across the card when the heatsink pops off.
also don't forget to clean the rest of the glue using nail polish remover or alcohol.