Those cheap, "high wattage" power supplies are pure crap. They will never put out that wattage at normal operating temperature. They watt-rate their stuff at room temperature (or less), "just turned on" degrees.
If you definately have the cash, get a nice one like the OCZ or even better a Turbo-Cool. Rated 510watts, at 50*C (or the "SLI" version )
If you definately have the cash, get a nice one like the OCZ or even better a Turbo-Cool. Rated 510watts, at 50*C (or the "SLI" version )