JmesMcM, I have what I believe to be the same exact tank as you. The All Glass 120 (4 X 2 X 2) with the dual overflows built in. Awesome tank by the way! I have the Amiricle wet/dry with the dual intakes to the bio-balls. My external pump is the Iwaki MD-40RXT, which was suggested by a very knowledgable fish and equiptment supplier. I forget the flow numbers of this pump, but Im sure it is published somewhere. From the pump I have a ball valve connected next(to cut down the flow a bit), then it branches off into a T where it is then sent back up the dual outflows back into the tank. I will get pics if anyone is interested.
This pump is too much for this application. It is too loud, but I guess all externals are louder than a submersable. It is warm to the touch, but not too hot. This is all I am using for water flow in the tank, it seems to be great for that. My sump is large enough for at least 3 submersable pumps, so I may go with a MagDrive pump for the main return. Right now I have the skimmer pump in there and thats it. Still plenty of room. This setup has been like this without problems for about 3 years. The pump is just loud. And I am cutting off too much of its flow with the ball valve. I suppose that if the flow was not cut down so much it would run just a bit cooler and quieter. At full flow it drains the sump.
Good Luck and ask any questions you'd like!