:::EDIT::: Doh, Kip beat me to it. But if you still feel like reading my novel...
First off, assuming your sump is below the tank... a loss of power will never flood the display tank. As soon as power cuts out, the return pump (in the sump) shuts down, and no more water is pumped into the display. What could happen is that the gravity-fed overflow could continue to drain water into the sump after the power goes out (and the return pump shuts down). When running, your sump should not be full. It should only be as full as necessary to keep all the hardware in the sump running. For me, this is about half full. Because my sump can hold an additional 8 gallons over normal operating volume, if I lose power, my overflow will continue to drain into the sump. But only as long as the water level in the display tank is above the overflow intake. The key is to get the water level in the display tank to drop below the overflow intake before the sump overflows.
It's not as difficult as it sounds, as long as your sump is a reasonable size for the tank. A 5 gallon sump probably wouldn't work very well on a 300g tank. I have a 13g sump on my 75g tank and I have plenty of room to spare.
Here is a good article on sumps. It's a good read, and should give you a good feel for how they work and why they do the things they do.
http://reefkeeping.com/issues/2003-01/gt/index.htm
