Take the measured wall power and multiply by the efficiency of the power supply in the test system to get the actual (continuous average) power used by the components inside the case.
example: 335 W at the wall outlet x 82% efficiency = 275 watts inside the case.
Assume that the majority of the power is used on the +12 V rails. Divide the power used by 12 V to get the DC current required.
example: 275 watts ÷ 12 V = 23 amperes on the +12 V rails.
Multiply the actual 12 V DC current used in the test system by a safety factor to determine what minimum rated +12 V capacity you want your PSU to have.
example: 23 A x 125% = 29 A minimum capacity desired on the +12 V rails of the new power supply.
You can multiply this by 12 V to get the required power rating (in watts) on the +12 V rails that sometimes appear on the labels instead of amperes. Note that this number is always lower than the total power rating of the power supply, which includes the other rails that are not taxed much by modern PCs.
In this case, an Antec Earthwatts EA500D, rated at 444 watts on the +12 V rails = 37 amperes, would be more than large enough for the system with a single GeForce GTX470 running the load that Damage tested it at.
i7-2600K, H70, P8P67-M Pro, 16 GiB, HD7950, SSD, 3 HD, Blu-ray, X-Fi Ti Pro, Antec P182, S75CF, 3007WFP+2001FP, RK-9000BR, MX518
i5-3570K, GeminII-S524, P8Z77-M Pro, 16 GiB, HD7770, SSD, 2 HD, Blu-ray, InifiniTV4, NSK2480, 55" TV; Asus UX32VD