Hello Matt,
I do not know what kind of body you use, and what kind of motorsport you shoot.
I personally was shooting Rally, with a 24-105 on a Crop body (40D), but if you are at a racetrack, it might not be enough.
I will assume, you have a crop body (Anything that is not a 6D, 5D, 1D).
On a Crop body, a 70-200 f/4 IS should be a very nice lens, for shooting Motorsport in the daylight, if you can get to about 40-50 meters from the cars (or closer).
For bokeh, if you need to keep a distance, bokeh will be never really nice, does not matter if it is a 2.8 or a f/4 lens.
You might also get a 1.4x Extender, but Autofocus will be a slower.
I woudl NOT recommend the 300mm, or 400mm fixed lens, as these are fixed, its not so versatile as a zoom, and you will want to zoom, as the cars are closing in.
As for IS. It can help in Mode 2, when you want to pan the camrea, it will stabilize in only one direction enabling you to get some really nice panning shots, but do not think, that this works wonders.
You need to learn to use it, then you will be able to really take an advantage.
Now if you have a Fullfreme body (6D, 5D, 1D), then I would advice you to get a 100-400, but that much more then $1000.
Well, thats my opinion, hopefully others will have similat, or totally different opinions. 