If its a single 1" drain u can estimate around 600GPH give or take.
If you have a 40 gallon tank you dont want to use a return pump that will come anywhere close to 600GPH.
Most people will agree that you want slow flow through your sump, 5-10x the size of your tank, 40Gallon*4=200GPH
There is a headloss calculator on the main page on the left. Depending on how many bends, or how far you go, this will reduce your GPH.
If you want a top notch pump look at the Eheim 1250. Should be just around right for your setup.
The biggest problem you get with going faster like a Mag 5 or 7 is you'll have water moving so fast u'll get tons of micro bubbles.. amoung other issues. Also if you start to come close to your overflows limit you'll hear a big waterfall effect...