It's about physics. Watt=Voltage*Amps(*efficiency). Typically in Physics or science class u don't need to consider efficiency but in real world, every electric-powered stuff is not 100% powered. For computer PSU, the power efficiency is around 80%, meaning that if u have a 500W PSU, the final output, positively, will be 500*80%=400W, and if the PSU in full load and the heat goes up, a brand new PSU may rest only sub-80% and as time goes by, the efficiency will decrease while the heat is slowly damaging the components.
So technically, u should consider your rig takes 60-70% of your PSU's capacity, which is the best way to protect the PSU and your whole rig. U know sudden PSU failure and the rig's power-off while in full load HARMS your rig a lot. So never be too scarce w/ your PSU and leave it some space.
As new CPUs and gfxs' power consumption goes up, nowadays manufacturers using 12V rail to supply such components, e.g your CPU Athlon II 240 has roughly 65W TDP, by using 12V rail the current output is 6-7 amps if efficiency is considered, plus CPU is using 1.3-1.4V low power voltage there is conversion between 12V and 1.3-1.4V and in such process some power has lost. gfx is using the 12V voltage so there isn't such loss of conversion but only max 80% power could be used by the gfx (the rest 20%-30% power is lost by heat, u would learn in physics)
GTS 250, based on the old architecture, has a 130W TDP, it's enough your 6-pin PCIE conductor (75W) and your PCI-E slot (75W) provide the power. But since GTX 760 has 170W, it needs another 6-pin to supply.