Ati Hd5750 power requirements

Your PSU will run a 5750 just fine. A 5750 only needs about 6 amps - around 70 watts.

If you look at the label on your PSU, it will tell you it has 3 rails each with a maximum rating of 18 amps. Your total is not 54 amps. That's 648 watts.

Below the current ratings, there should be a line telling you the max total 12 volt power in watts. Divide that number by 12 to get the max 12 volt amps. I'm guessing that it is around 40 amps. That's a reasonable number for a pretty good 550 watt PSU.