I used to have auto-topoffs on both of my saltwater tanks. While I agree with Matt to some extent, it's essential if you're running a lot of lighting over coral that don't like salinity shifts. For topping off a freshwater sump, it doesn't seem as important as long as you have it covered and it's of sufficient volume to let you top off once a week. I never have to top off the sump on my 180 as I do massive changes every 7 days or so.
Not sure of your configuration, so it may be necessary though. On my old 26 bowfront, I had a 10 gallon sump. I had a 250W metal halide as well as VHO actinics over it and 4 powerheads running, so you can imagine it evaporated a ton of water. Before I set up the ATO, if I missed two days, the return pump would suck air.
Really simple. Got two float switches online and connect them in series to a short extension cord. You use two for redundancy - one higher than the other, at least one encased in a small plastic jar (to avoid snails keeping the float stuck down).
If you use a small pump, you shouldn't need a relay. I used a Micro-jet on my 26 gallon setup and an Aqualifter pump for my desktop nano. For the resevoir, I used a 5 gallon bucket with RO.
Here's a site that just found with almost the exact setup I was running. Do the research on the relay though to see if you guys want it for safety. Worst case scenario is that you burn out the wiring on the float switch - which would shut the system off (or on, depending on how severe the short was).
Their prices are a bit high, but they offer almost the same parts I was using.
http://www.aquahub.com/store/diygaquarium.html
Good luck and be sure to post pictures.