I shoot HS sports with a Rebel XTi. It has served me well, once I got some fast glass, but I’d like to upgrade.
My main reason for wanting to upgrade is that my XTi hasn’t performed so well during football season. I use my 2.8 70-200L at 320 shutter speed w/o a flash at 1600ISO and have mixed results. I have a monopod and I’ve used an L bracket and hose clamps to mount my speedlite low for this coming Friday. I’d rather not use flash but I’m hoping it might increase my rate of ‘keepers.’
So what I want when I upgrade is a better AF- my XTi hunts in the dark- and better ISO performance- so that I don’t have to use a flash during football season.
Is a 40d going to offer a _big_ performance gain in those areas over my XTi? What about a 5d? Or a good used 4MP 1d? I’m afraid that the new 50d and the new 5d replacement are going to be out of my budget.
FPS isn’t as big a concern for me and I don’t think that losing the 1.6 crop factor would be either since, at least during football, I rarely go all the way to 200mm.
Thanks for any suggestions or help or information.

based on the review from member here the AI Servo performance ranging (best to worse) would be:
1D/Ds series
40D
5D

then again fps would be one big consideration for sports as you can miss many situation with low fps / one shot.

If sports is really the main use of the camera, and you want to maximize
performance and value for your money, then get a used 1D mkII, if you
can stretch your budget to around $2000.
This camera will give you excellent AI Servo tracking, and it has enough pixels.
The high ISO performance is undoubtedly better on a 1DIII, but you will not wish
to spend that much money.