There are psu calculators out there. Unfortunately most of us can't accurately determine such factors as cpu usage, capacitor aging and other inputs to the models.
I find it simpler to use a rule of thumb.
The psu requirements are mostly determined by the graphics card/s.
Assuming a normal complement of hard drives, fans, dvd' etc,
Here is what EVGA recommends for their graphics cards:
GTX550ti needs 400w with 24a on the 12v rails plus one 6-pin PCI-E power lead.
GTX560 needs 450w with 24a on the 12v rails plus two 6-pin PCI-E power leads.
GTX560Ti needs 500w with 30a on the 12v rails plus two 6-pin PCI-E power leads.
GTX570 needs 550w with 38a on the 12v rails plus two 6-pin PCI-E power leads.
GTX580 needs 600w with 42a on the 12v rails plus one 6-pin and one 8-pin PCI-E power lead.
GTX590 needs 700w with 50a on the 12v rails plus two 8-pin PCI-E power leads or 4 6-pin power leads.
It is not wrong to overprovision your psu .
But... do not try to economize on a cheap psu.
Get a quality unit, and you will get what you pay for.
My short list of quality psu's would include Seasonic, XFX, PC P&C, Corsair, and Antec.