If your system is as basic as you said, the 400w PSU should provide enough power to your 7750. But if you are willing to spend some money to be safe, get a good 650w PSU in case you buy another 7750 to crossfire in the future.
And if you want to crossfire be sure your motherboard supports it; have another graphics card slot on it. Mind the power connector requirements for your graphic card(s)
He could get away with 250W if it were reliable. His entire system would draw about 200W on load. Crossfired 7750 would only pull around 300W, so if he crossfired he should get a 400W. A 650W for crossfiring 7750s is overkill, a power supply is most efficient in 75-80% load. A 650W power supply would only have a 50% load.
My i5 2500k and 7970 only pull 350W (I measured from the wall with a Kill-A-Watt) and I get away with a 500W PSU. People always overestimate the power supply needed in these forums.