It's a little hard to say because that video card "recommended" amount is most likely estimated full build amount, which may not take into account certain parts. Look for "power consumption" of the GPU, add that to the rest of the parts, then I'd recommend about 100W headroom from there. So say the GPU power intake was 200W, the rest were about 200W, i'd say a 500-550W would do you just fine. 400W is basically unheard of for consumer graphics cards, as new ones are just starting to go into the 300s, so it's most likely a build recommendation haha.