According to a press release by Enermax, your power supply may or may not be able to support one of the upcoming Haswell’s lower power consumption features: the advanced power consumption C6/C7 modes Intel is touting. According to Intel, while in these ‘power conservation’ modes your processor will draw as little as .05 Amps. To actually achieve this remarkable efficiency your power supply will have to be able to support low load modes.
Enermax has a list of compatible computer power supplies on their website. While they have cautioned that not all power supplies will support zero load and offer stable voltage levels under low-load configurations, they have also acknowledged that some already on the market actually will (look for one that can output 0.05A on the +12V rail).
Because power supply makers may not have tested or spec’d out low-load efficiency, you may want to err on the side of caution and buy a new Haswell compatible power supply.
For more information read the Enermax article.