I think you are exceeding your PSU's limit. Your GPU will run on a 400w PSU but you need to take into account that your other components (CPU, RAM etc) take power aswell. You may need a new PSU
I dont think so, because the computer used to run with the same components (except for the graphics card I try to make work) on 220W psu.
cpu consumes only about 60W and the ram consumes a bit less.
I dont think the power of 400w psu is a problem. The AMD site says 400 is a requirement for psu an the card is low profile. Is it possible that there is something else that may prevent the computer from using the card.