Good PSU remains efficient even at high loads, produces stable voltages at all loads, and typically has higher amperage than cheaper units (important). Overall efficiency is also higher, saving you a few bucks on your bill.
Better units usually also have multiple rails (circuits?), meaning there's 1 rail dedicated for graphics hardware (or was it CPU?) and the other(s) for all other components.
My brother had crappy PSU. He installed a graphics card, upgrading from onboard. 1 month later his HD died...
Anyway, with those specs it's gonna FLY!
