I am planning to build a new pc and I am not sure how big of a PSU do I really need.
Components: Mobo-ASUS SABERTOOTH 990FX R2.0
CPU- FX 8350(oc3d to 4.6 GHz)
CPU cooler- Corsair H100i
GPU- Sapphire r9 280x VAPOR-X
Ram- Kingston HyperX Blue 1600MHz
SSD- Kingston V300 120GB
HDD- Western Digital Caviar Black
Case- Cooler Master 690 II Advanced
If you plan on overclocking, at least a 650w. The power draw for that system is about 500w. If you are going to crossfire, check the prices on 750w - 850w and buy one from a good brand because it'll be cheaper to buy the Psu now, than replace the Psu down the road.
:edit: The Corsair CX series should only be used in budget builds. Any of their other lines are solid, but the CX(and GS actually) use cheaper quality components that aren't as reliable. Look for Seasonic, OCZ, XFX.