hasn't been edited since early/mid 2012 but it's good data and still valid for the most part
5450 is 15w max
GT210 is 18w max
8400gs is 20w max
but that doesn't address performance, one of those cards could be 5 times faster than the other at the same power level so it's a whole different set of research to figure out if they are good performance wise
and a third discussion altother for hardware reliability and driver issues
oh and a forth discussion when it comes to price.
in short I'd avoid the 8400gs and pick the 5450 or gt210 based on price, physical size, preference between AMD/NVidia (my bias is for the 5450 for multiple reasons but its not the end of the world if you got GT210)