The 3650 is rated at only 65w and the 5500 series at 40w by ATI/AMD itself. It is very unlikely that either of them are taking 300w/400w. Some marketing guy probably got too enthusiastic.

For reference as to how high 400w is, note that the current fastest GAC by ATI, the 5970, with 2 whole GPU running together in it, still only takes 320w max (And only 50w at idle) A 5500 with the small cooler it has at 400w would melt a hole through your motherboard, your desk and possibly your floor.