pump question

[ricker]

im building a fuge out of acrylic and since my tank is not drilled im buying an overfloe but my question is what size and kind of pump should i get for the return line, it will be pumping water roughly 3 and a half 4 feet, and i cant decide if i want my overflow at 300 or 600gph, could someone let me know what pump to get, my tank is a 55g could anyone tell me which would suit my needs properly, 300 or 600, and which pump

 

[fender]

You want the flow to be slightly less than your overflow.
For the 600gph you would want a pump that flows around 550 at the outlet. You have to take head height into consideration in this. For example a pump that flows 600 at the pump may only put 350 at the outlet. Check the distance from the pump to the tank in feet, add a foot for every elbow and find a pump less than 600 at that height.
So....
Assuming the top of your tank is 4ft from the pump and you need 3 elbows you'll have a total head of 7 feet. Find a pump that has the flow you want (550gph) at 7 feet of head height. Install a valve or tee to allow the pump to be throttled back in case you overshoot you estimate.

 

[fender]

I realize my answer is a little ambiguous.
What are your needs for the tank?
IMO I would go with the bigger overflow.
As far as pumps, do you want internal or external? Heat can be an issue with some internals.
This is what I am looking at right now for my tank (a 65gallon - debating internal or external).
For internal - a mag 7 or 9 depending on your head height (with the 9 you'll probably need a valve, SCWD or sump return).
For external - Velocity T2 or T3 depending on your head height (with the T3 you'll probably need a valve, SCWD or sump return)..