PSU?

I had a quick go at building a pc for my friend but I don't know which PSU to 
How do you know how much you need?
I was trying to keep it under $100 AUD so the first PSU would be the best
Would it be better to downgrade to 6GB RAM and get the 750 PSU?Antec Neo Eco 620C 620W Power Supply [NEO-ECO-620C] - $89.00 : PC Case GearAntec High Current Gamer 900W Power Supply HCG-900 [HCG-900] - $157.00 : PC Case GearAntec TruePower 750W Blue [TP-750-BLUE] - $145.00 : PC Case Gear
another build I did in AUD
http://screensnapr.com/v/Bs8izo.png

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