The card will not consume 400watts.... It actually only consumes 39watts max and around 10 watts idle....

The 400watt your reading is the minimum system requirements for the entire system (i.e you ideally want at least a 400watt power supply in your computer)...

The reason its a higher requirement is because its a newer card and its expected to be in a newer system with more power demanding hardware... Though saying that i ran a PSU calculator test and interestingly when i set the calculator to have roughly the same card as the one your thinking of getting the PSU requirements came back lower than your old card... in short your new card is more power efficient ;)

heres the link to the PSU calculator i used incase you want to see for yourself ;)
www.extreme.outervision.com/psucalculatorlite.jsp