Get the OCZ ModXstream Pro 600w (this is what I have on my system) or the Cooler Master Silent Pro 600w. http://www.hardwaresecrets.com/article/936
Edit* FYI I have the same video card, HIS Radeon 6770, on my i5-2500k system OCed to 4GHz on stock voltage with Gigabyte Z68. Wattage for my build is about 450w. Since you got the FX4100, it will consume a little bit more power but I'm sure it won't be over 500w. So 600w PSU will be good enough even you decide to Xfire wih another 6770 in the near future.