Core i5-2500k and HD 6850 are both pretty low on power consumption, lower than you might expect. At stock, this is the power draw:
Even with some pretty hard overclocks making everything draw 50% more than stock, you're looking at under 500W for a whole system if you Crossfire HD 6850s. Seeing as real-world power draw is under LinX + OCCT load, you don't even need a 620W if you want to Crossfire later, even though that's not a bad idea.
If on a budget, I'd take the Antec Earthwatts Green 500D for $50 after promo code:
Something a little better would be the XFX Core Edition 550W (Seasonic S12II Bronze 520/620 internals) for $70 before $10 MIR:
If you want to spend up to $90, that's how much the XFX Core Edition 650W and Corsair TX650 V2 cost before rebate. Both those units are Seasonic SS-***AT internally--very good quality. That kind of unit will run something like Crossfire HD 6950 easily.