Here are the ATI recommended power supply requirements for the ATI Radeon HD 5750 video card:
450 watt or greater power supply with one 75 watt, 6-pin PCI Express® power connector recommended.
600 watt or greater power supply with two 75 watt, 6-pin connectors for ATI CrossFireX™ technology in dual mode.
Since you intend to use only one video card, the Corsair VX 550 would be an appropriate choice. It can easily handle an overclocked system with any single video card made. Corsair has a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty.