Try finding a 360 forum, I don't know of a better heatsink. But what is wrong with the factory heatsink, millions went into research and development for the 360. The people who built it knew what they were doing. If it were me, I would clean the heatsink and the gpu core and apply a high quality thermal paste.
EDIT: it seems I am a n00b
EDIT: it seems I am a n00b
