I use a Hagen Aquaclear 901/110 powerhead on my present 180g with 33g sump system. They push 900 gph at 0' head. My head is at 4' approx. (makes me sound short
) so that calculates to about 750gph, if I remember correctly. But I have two 6" corner overflows, not just one, for a total of 24" linear overflow. You should determine how much gph your overflow can handle (there is a calculator on Reef Central), factor in the head, and then size your pump accordingly so it doesn't exceed the linear overflow capacity.
HTH,