Head loss calculator
http://reefcentral.com/index.php/head-loss-calculator
Melev's Reef - Sumps
http://www.melevsreef.com/links.html
"As you suggest a sump for such a small tank I'm throw." What?
A four bulb T5 would work well or I'd go with a 150W MH. Even a single 250W for a shallow nano-tank would be overkill.
Eheim pumps are an excellent small pump.
A two drain overflow is typically called a Herbie Overflow and will fit a 20gal tank much easier than a bean-animal which are pretty elaborate and require three holes.