do you think 500-550 is pushing it too far?
A 500 - 550W power supply is even better. The efficiency wil be higher (usually the power supply achieves its highest efficiency when delivering between 40% and 60% of its maximum capacity). The disadvantage is the price of a higher wattage PSU. Anyway, at the long run it is a good idea because you will save money on your electricity bill and you will have a higher headroom for upgrades if needed.