My GTX 760 OCed from factory (1178 Mhz on core) was using max 196 Watts. I checcked the GPU BIOS. Yours wont use more. Add your CPU, 100-150 watts maybe. 350 total. Couple harddisks, fans, 400 w total.
You don't want to run your PSU at 100% all the time, which is why manufacturers suggest a bit of headroom. I.e. 500 W.