Depends on the power supply.
Models like Ultra, Coolmax, Powermax, Rosewill , Raidmax etc... Always rate their powersupplys at max output. They will put out 500watt but only for a few milliseconds. They will run continous with an output in the range of 325 to 350 watts.
Models like Corsair, Seasonic,Antec will run continously putting out 500watt and some will go as high as 575 watt before shutting down.
So we need to know "WHAT" power supply before we can answer your question.
Edit: If you are looking for a cheap Good( not Great) unit the Antec BP500U has been rated by Johnyguru as cheap but good and stays within all ATX specs and puts out 500watts.
I totally agree with Rick. Lucky for you you don't have any ultra high end stuff in there chewing up power. With no OCing I put your power usage around 300W, and probably lower then that. Any PSU that can output 25-28A will be fine. The trick as Rick pointed out is finding one that can do it long enough to handle your gaming sessions.
One of my systems has an OC'd Q9550, 4 GB RAM, a GTX260 - a card with similar power requirements to yours, a Gigabyte EP45-UD3P motherboard, 3 hard drives and an optical, and a Soundblaster card all powered by a Corsair 750TX.
Running 3 instances of Prime95 to load the CPU and 3DMark06 to load the GPU, it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% efficiency, the system pulls 300 watts from the PSU.
You have a dual core chip so your power drain should be at least 30 watts (2.5 amps @ 12 volts) less unless you succeed in unlocking the CPU.
I'd say that, while you do not have plenty of power, you do have enough.