To get an idea of the PSU needed, often the Video card maker will put the minimum wattage required for that card.
for the card you chose
Quote:
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System Requirements 450 Watt or greater power supply with 75 Watt 6-pin PCI Express power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX technology in dual mode)
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Myself, I think it's better to err on the side of bigger and you won't have to worry if you decide to upgrade your card or other components.