Hi im looking at an XFX Double D R9-280A-TDFD Radeon R9 280 3GB 384-Bit GDDR5 PCI Express 3.0 CrossFireX Support Video Card and the websites say the minimum wattage is 750w. Yet any wattage calculator that ive looked at said the end wattage for the pc would be much lower. Which is true? Also does this mean my power supply would have to be min 750w? (it's my first time building so I'm being extra cautious)