keep in mind, if your using a tri-pod/monopod then spending the extra coin for the IS is pretty much a waste of money. there is the mode for panning, but generally it wont assist much in that situation. if you will be doing handheld shooting with the lense, the IS can save a lot of shots when light is gonna factor in.
as for f/2.8 vs f/4...no brainer, 2.8 all the way if you can afford it. if not, the 4 is a very nice alternative. dont even factor in the weight of the 2.8 vs the 4 as IMO you will eventually build up the muscles that may fatigue in the beginning (if there even is any fatigue when you first get it), the 2.8 isn't really that heavy in the end. i have shot many a day long amatuer basketball tourneys with the 2.8IS and after a couple weekends your body adjusts and personally i like the weight of the 2.8IS, gives a nice solid steady feel to it when handheld shooting. and as dr.bear above said...
You know you want the 70-200 2.8 IS. Click "buy now". Hurry.