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.
5770 Crossfire System Power Consumption
Idle = 109W Load = 324W
This system is using an i5-750. But I think that shows that 6770s don't use a lot of power, so the only reason to go above 450W is to overkill or to give yourself lots of upgrade options. Or if higher wattage units are better priced and higher quality/more feature rich than lower wattage units.
Thanks alot guys. I should mention I live in Australia so I am quite limited to decently priced parts. I found the OCZ ModXstream Pro 600w for $85 so i am gonna get that i think.
I'll probably be upgrading to the next generation video cards when they come out. as theyre supposed to be more power efficient so I should be alright with this psu.