You don't want a pump bigger than your overflow can handle.
eek! yet another VERY important detail I forgot to mention! Thanks for chiming in Ellisz! (thats why it's always good to get more than one opinion!)
If you have 3 other powerheads in the tank that you plan on keeping (a good decision) then you won't need 750 GPH
coming out of your sump. Figure out what your overflow is rated for (the larger hole of the two) and then look at that mag-drive flow chart. (Sorry, gonna make you do the figuring and decisions on your own. :wink: It will help you understand how and why your sump works the way it does so you can troubleshoot and fix/adjust as need in the future) and get a pump that won't have a higher flow rate at the 4' head height you'll have than the overflow can handle.
IOW: (this is an example, and the numbers do NOT match your tank... cause I don't know what the numbers are :wink: )
Lets say your overflow can handle 400GPH.
Lets say you find a pump rated for 600GPH, but only pumps 350GPH @ a 4' head hieght, then thats the pump you want. But if you find a pump that's rated for 600GPH but pumps 500GPH @ a 4' head height, you DONT want that pump, because it will pump more water into your tank than your overflow can handle. (and thus your main tank will overflow onto the floor!)
Are things making a little more sense yet? And I will second the vote to purchase one online. I bought a Mag7 for I think $50+/- for my 55. (closed loop, flow only)