I'd get a Mag 5 or a Mag 7. Mag 5 is probably bare minimum at 5', you would get 250GPH and I hear 10x tank turnaround per hour is optimal. Mag 7 will give you 420, which you might want to pipe down a bit with a valve.
I run a Mag 12 myself and I am very happy with it. Mine does run a bit warm though, raises water by about 10F over ambient.
NIKON pimp #017
Sump pimp #014