I wouldn't even consider running a 5970 on a 550W PSU. His processor has a likely TDP of 95W. The 5970 a peak power draw of 294W. Add other pieces of hardware and you're likely reaching between 410 and 430W peak load. If you put a 550W PSU under that stress, it'll probably buckle under the load and take other things with it especially if it's not a good brand. The closer it gets to its limit, the less efficient it will be at providing clean power.