You won't have any problems with your power supply. I have a 550w power supply with 14 and 15 amps on the 2 rails and i run a vapor-x 5770 overclocked video card and my CPU is also overclocked to 3.4 from 2.67Ghz.
Buy the card and enjoy it. It will be worth it .
And i know that my PSU has a low amperage for it's power but i have it for 2 and a half years and it was not a new model when i bought it.