I have had mine start at -20 actual temp, with wind chill it was colder that that, last winter in the [FONT=arial, sans-serif]Adirondack Mountains. They will start when its cold. The block heater makes the oil warm so it flows easier reducing engine wear and It make the heat blow hot faster, in addition to making it start easier. So I would say the temp it is needed at is personal preference.
In my experience gelling usually occurs in the small orfices ,fittings,filters or injector lines, wherever you have a smaller amounts of fuel, so the block heater helps on this because these areas are generally on the engine. I pesonally have never seen a fuel tank gelled but I'm sure if it was cold enough it could happen.
How hot do they need to be, I think that is a matter of opinion also. I like it to be on hot idle when it starts and the 1000 watt will generally do it in my area if plugged in long enough but not always.[/FONT]