Not all ATX PSU's are compatible, there is 20 and 24 pin PSU's. (24pin PSU's are for PCIE motherboards, and are called ATX2.0.)
Those 2 should be compatible, but before you get that PSU, you should know that generic PSU's are terrible. Their reliablity is not great, as they die early, and when they die, they could even take out your whole system. They also fluctuate alot, as the voltage on the lines, can go dangerously over or under, and cause restarts. That 400w is not measured in a real world enviroment, and is a peak, so it could be 250watts sustained, and for a split second hit that 400watts, and they call it 400 watts.
Post all your system specs before buying BTW. and we can make recomendations.
Cisco CCNA, Comptia A+, 1/3 through CCNP