The Corsair VX 550 watt power supply mentioned by ulysses35 would be an appropriate choice for a pc system with one ATI Radeon HD 5870 video card. Corsair has a reputation for high quality psu's. They consistently earn high marks in technical reviews. They are stable, reliable, and come with a 5 year warranty.
The determining factor for case selection will be the ATI Radeon HD 5870 video card. The card is 11 inches long. Add a little more for the power cables. That pretty much rules out most mid-tower cases.
The new Lian Li / Lancool Dragon Lord cases have 11.417 inches of space between the hard drive cage in the bottom front of the case and the rear panel. Ventilation, airflow, and cooling are excellent. The cases are available in a variety of configurations and prices: