lol.. As MuFu said you can stick it in the frezer for a bit then take it out and put a flat screw driver or a butter knife between the card and the HS. Make sure to put something like a rag between the card and the knife. Twist it gently and it should pop right off. It always works for me.
Just becareful not to use to much force. If it wont budge on one corner, try another. GoodLuck!
Use a fillet knife or other very sharp very thin knife. A razor blade would work if you had one large enough to get under the heatsink. Simply work the edge of the knife/blade under the edge of the heatsink between it and the corner of the GPU core, and then twist the blade gently to pry the corner loose. Work from all four corners if you can, not prying too hard at any one of them.
Doing this should free the heatsink without using too much force anywhere. Most GPU coolers just have a scattering of thermal epoxy in the center, so it shouldn't be too hard to remove. Just be patient and keep working it loose... better to take 10 minutes and get it off nice and clean than to do it in one four second jerk and remove half the CPU in the process.
------------------ Only 30,000 to 40,000 genes in an Einstein, a Michael Jordan, or a Bach? Boy, can that God guy write tight code or what? - David Rudloff