It's fairly easy actually if you think about it.
Since your CPU cannot be OC'd, by itself, it will consume something like 95w under the worst case scenario. The HD 4870 will likely take around 160w-170w under full load while hard drive, dvd drive, case fans etc will prolly add around 50watts.
So you can easily run this entire system off a Antec EA 380D under full load with no problem at all.
Most of us would recommend units between 500-600watts from companies such as Seasonic, XFX, Antec, and Corsair (along with select others units) because these are quality units of quality build that can do its rated wattage.
Not to mention, it's not about the wattage, its all about the amount of power that a PSU can put to the 12v rails. Not enough power = unstable PC = generic PSU's that put their max power on the label and not the continuous power such as more high quality psu's.