Begin by following the excellent existing guide on opening this model of iMac, thanks to Brittany, located here.

STOP at step 11 of her guide, "Lift and remove the display panel." You do not need to remove the HD to replace/upgrade the video card.

As Illustrated here, I chose a less-expensive alternative to the $15 suction cups to lift the glass. For less than $5 I got a pair of 3M "Command" adhesive coat hangars. They proved more than strong enough. The easy release feature came off cleanly with zero residue.