PSU Wattage Calculator
If you didn't change the system load option, then it is at 90% load. Your CPU uses 65W peak and your GPU uses 105W peak. A decent 550W PSU is overkill for your systems, high end 550W power supplies (such as the COOLER MASTER Real Power Pro 550W) can run your system but with two 9800GT cards in SLI.
The calculator isn't 100% accurate, but it gives you an idea.