There called snatch blocks.
One thing to consider when choosing a winch is the type of wheeling that you do, and type of vehicle. lots of mud, you are going to want a larger winch cause when your recovering a vehicle, you have the vehicles weight as well as the suction created with the mud and tires, so... a 4000 lb vehicle will require about 8000lb's of pulling power.
if you wheel more on the open trails and some rocks, you can get away with a smaller winch, but I would avoid trying to pull out some full size rigs!
If you perfer to wheel a stock, or close to stock jeep, you will need less of a winch cause you vehicle weights less.
Unless you are considering competition, I would not be too concerned with the line speed.