I'd suggest hiring a car. It will be cheaper than taxis to town and back. Generally it's pretty easy to go through immigration at DFW, so it shouldn't take all that long.
When you leave the airport, take the southern exit and head east along the interstate. There's an off-ramp just near Dealey plaza and there's a public carpark quite close to the Depository. You should do the trip in 30 minutes.
You buy your ticket downstairs and then go up to the Sixth Floor. They give you an audio guide and you take yourself around. It is very well done. Various displays cover the different theories of the killing and its investigation. The area around "that window" is set up just like it was on 22 November 1963, though it has a glass barrier around it. You can look out of an adjoining window, to get an idea of Oswald's view. Gift shop at the end (of course).
Then walk out into the plaza and look at the memorial, and the X that is painted on the roadway at the approximate location where Kennedy was shot.
A few blocks away, there's also an official JFK memorial, which is comprised of rather dull concrete blocks in a geometric pattern. Don't go out of your way to see it.