1. The power supplies 12 volt rail is limited to 408 watts not 500. The total is not as important as the 12 volt rail. I have seen 550 watt power supplies on the market with 24 amps on the 12 volt rail. We have less and less of those awful things every day thankfully.
2. My concern is the variation from system to system. Some boards are more efficient than others and that card will take over 250 watts is subjected to furmarks thus the warning.
My personal Media Center pc with a i5 750 + gtx 650 ti has a hard time breaking 150 watts(despite the TDP of all parts. some parts stay well under while others can be over under certain loads. TDP is not a be all and end all) at the wall(I can get it upto 180 if i try idle is in the 40ish area). Yet another system of the exact same specs may pass 200+ on load. No 2 systems are the same(even among cpus some run at lower voltages than others).
I would much rather recommend as safe an upgrade as I can. Even power supply calculators overestimate.
MichaelEpicBeast :
But my cpu needs 125 watts.
the TDP of 125 watts is shared by a full set of those cpu's(not even just your model number) So some will take exactly 125 while others may be well under or slightly over.
I would highly recommend a watt meter from the hardware store. They sell for 20 dollars many places and tell you what the system is taking at the wall. Since the DC power use is lower, being under the rated power supply output means the computer is drawing even less. All power supplies waste some power in the form of heat.