Quoted from here http://hothardware.com/Reviews/AMD-Radeon-HD-7770-and-7750-Affordable-DX11-GPU-Review/?page=2
With its pared-down, lower clocked, GPU, the Radeon HD 7750 requires less power than the Radeon HD 7770—55w vs. 80w. As such, the Radeon HD 7750 does not require and additional power feeds. The standard 75 Watt power budget provided by a PEG slot is more than enough. The Radeon HD 7750 also has a much shorter 7” PCB and a single-slot cooler design.
The pci express slot on it's own provides enough power for the 7750. The 400w requirement is overall power consumption for the whole computer with this card.