I would say get as large a pump as your overflow is designed to handle, and 10 X the tank volume is a general rule of thumb.
Also, I've been thinking of trying something on a new tank I am considering.
Instead of just having the return line run up to a single or dual spray bar, I'm thinking of building a rigid tubing system out of PVC or some other material, that would run the full legth down the back corner of one side of tank, acoss the back bottom behind the live rock, and back up the other side back corner. This tubing structure would either have positioning jets or small holes drilled at various point to create many points of water circulation.
Run a tee off my return line and feed the top water surface spay bar and this tubing system. Would take a pretty good pump to do all this - huge pressure drops across this thing.
I've only thought about it and have not put anything on paper yet ..... may be a silly idea .... not sure yet.
I hate the look of powerheads in tanks, but this contraption may end up looking even worse LOL