The 5670 amd reference board is claimed to use 64 Watts.
Depending on the cpu i think you should be safe taking around 120 Watts in account.
for other stuff like hard drives opticals etc, sit around 100 to 150 watts depending on how heavily you configure your rig.
64 + 120 + 150 = 334
So go for a 350watt minimum.
Offcourse like others stated it's never bad to way over that, the psu will never use more then the system demands.
Just look for a good brand with 80+ certification.
Personally i use coolermaster, and that psu is now over 5 years old and still rocking.
Hope you have enough info now, good luck!