For a single 7850, the 520W Seasonic (or any other 500W) PSU should be more than adequate..
Additionally, u could use one of the many PSU calculators available online to estimate
the 'wattage' u would require. Here are a few links:
Cooler Master calcAsus calc
"be quiet!" calc
TPU has a thread
called "PSU guide". It has nice general info..