I really think you have answered your own question. You are on the cusp of not being able to handle the power requirements of your system and you are nervous. The question has already been answered according to experts but you are still unhappy. The answer? BUY A BETTER AND MORE CAPABLE POWER SUPPLY.
Put out the extra cash and buy a good silver/gold rated power supply. It will cost you more but will give you the peace of mind that you obviously need. 600W silver would be MORE than perfect for your situation, and you would never have to worry about overheating/overload. If you plan on doing dual GPU's in the future you could even get an 850-900W Silver/gold, though if you don't plan on doing this in the future this would be overkill to the max.
I hope this helps.