overflow speed in 55gal tank

I am looking at overflows for my 55 gal tank and I am seeing 600 GPH 1000 GPH and 1200 GPH on evilbay and was wondering if the 600 is all I need for a sump/fuge or if I should go bigger.

I am running a rio 2100 for a return currently.

 

i can tell you that on my 30 gal tank i am running a cpr s50, it is supposed to do something like 300 gph, and my return pump is 500 gph so i have to tune down my return pump to keep the overflow box from getting to much air in it. so if you already have the return pump then i would get a box to match the pump

 

the tanks internal flow is more important than the overall flow back to the sump

 

i have a coralia2 and another like sized pump in there now keeping a decent circular flow. I used the DIY overflow pvc plans but the inlet on the PVC doesnt pull enough water in for my tastes and leaves floaties in the tank that should go into the sump.