I have one of those tank heaters, but I havn't installed it. I think mine is around 1200 watts. I also have the one that goes into the block, which I havn't installed yet either. However, I do have one that a previous owner installed in the radiator hose, which doesn't seem to work that well.

Battery heaters will also help assist with cold weather starting. The best suggestion I have for people, is to make sure that they have a good starter, and it should be the updated model. Once I replaced the starter, I was able to start the car when it was in the 20s with no problem at all...