jonnyG recommends using something flat and wide to bend the casing up (won't damage the casing so much as when using a screwdriver).
No, the casing has nothing to do with the heatsink fixation, but you won't be able to use that plastic cpu retention thingie. Usually it is not needed as the cpu sits firmly anyway. I first took off my casing, installed a gfd, then put it back on (notched it for the gfd and installed a fan on the back of the cpu, which I attached to the plastic casing)- didn't help my overclocking, but I can use the cpu retention now.