Without knowing the specific model of PSU its hard to make a definitive suggestion.
400W is a pretty small PSU. If it's a crappy one that figure could realistically be under 300W. If it's really crappy, you don't want to use anything like that kind of power
In terms of graphics cards, the best graphics cards you can buy that only use 1 PCIe 6pin power connector are the HD7850 and GTX660. Both of those cards have a recommended PSU capacity of 450W, which is usually overstated but if the quality of your PSU is questionable it's probably best to adhere to it.
In the middle ground there are the HD7770 and GTX650Ti, both good cards that also use a single 6pin PCIe, but both cards still recommend higher power capacity.
At the lower end there are the GTX650 and HD7750. The latter doesn't even need a power connector, and both draw very low power. I'd imagine that either of these would be 100% safe.
A good entry level PSU will power any of the cards I just mentioned without any issues. Where you are actually capable of landing is really dependant on what PSU you are using and how confident you are in it.