The RROD guides i've read all suggested to remove the 'X clamp/s' by twisting a screwdriver against the pcb to release the clip. this may be ok for the skilled and adept with hand tools, but highly dangerous to both theless well equiped 'engineer' and the precious motherboard.
i find a far easier and better method, which is also way less risky, is to work on opposite corners, if you insert your screwdriver blade into the space in the clamp and lever the clamp against the pin, pushing the clamp to wards the opposite corner, then the opposite corner clamp will release with minimal risk of slipping or damaging the pcb, and the 'engineer' is safer too.
i find the best screwdriver to use is a larger neon, the blade is just right for the job
when refitting the heat sinks, a dab of the past on the washers will help keep them in place