your PSU is only 115W? definately replace it its the most likely culprit and it sounds rubbish anyway.
A cheap PSU ought to do your PC fine, a $30 400W generic jobby. Obviously the more you spend the better quality you get, more stable voltages, more power, more reliable parts, quieter cooler running.. etc
Intel core I7 920
6GB OCZ platinum 1600
Samsung 1TB F1 Spinpoint
Samsung SM2443BW 24"