i have noticed that quite a bit of people have plumbing issues, which have a lot to do with flow problems
one thing to remember is that the more bends (elbows) that you have will cause less flow (higher headpressure), so the less bends that you have the better off you will be
i also see a lot of people using flexible PVC, which is a great idea for those odd bends, but this also causes head pressure
another thing to think about is when buying ballvalves (which i recomend for cleaning and replacement) is to get free flowing valves, basic valves restrict flow, ie headpressure
the point is when buying a pump keep this in mind, you might have to get a larger pump than you thought because of the head pressure
one thing that i have done is to take the 1" PVC (for my return) and actually bend it to go arround the top of the tank so i didnt have to use elbows(my tank isnt drilled), and in other places i used 2 45deg's to make the bends easier on the pump, so the pipe is at angles instead of making hard turns to end up in different spots in the tank, this has saved me some money and headpressure i have about a 10.5-11 foot head w/o any bends and with the bends that i have i think it is around 15 foot +/-
another thing that i will be using, when i get a chance to get, is a SQWD which is a very cool device which helps to mimic the natural movement of the ocean, and which IMO are great with closed loop systems
ok, i will shut up now
TD