400W is not the only thing what you need to know is your PSU from reputable manufacturer? and how old it is?
reputable manufaturer usually marks psu not by max peak wattage but w that can be constantly produced.. also age is important because capacitor aging reduces performance.
if your psu is not some no name and not older than 2-3 year Geforce 650 should run without any problem.
I noticed lots of people recommend to buy PSU that is overkill for pc its better to buy quality product than have more W.. to buy PSU with more W just "for future upgrade" is bad idea.. because pc architecture is ever changing (new CPU generations, new sockets, in this case PSU cap aging, etc) you buy PC for now not future.. when you need faster pc or more power you look to replace that part or buy new pc.
and if you dont know what youre doing i would advice to be very carefull with any tests that tries to burn your hardware.. most games will never load your hardware 100%