Hmmm. I'm tempted to say, why not try it and see?
But instead I'll say, in the world of sports shooting, we say "Never sacrifice shutter speed for ISO," and "A sharp noisy shot is better than a clean blurry one."
What that translates to is this: Use the lowest ISO that gives you the shutter speed you need, but no less. I would think that guideline would be equally applicable to what you're doing.
The good news is you are fortunate enough to have two of the very best cameras available in terms of high ISO performance (as well as in terms of just about everything else, for that matter). So don't be afraid to use it, and
learn how to use it. That means proper exposure and, when necessary (which will probably be less often than you think), judicious use of noise reduction software.
Remember the old guideline about 1/focal length as a reasonable
minimum handholding shutter speed. That means if everything goes right using good technique... bracing, breathing out, squeezing the shutter, etc. On your 1D that also means 1/focal length x 1.3, because of the crop factor. And again, that's
minimum. More is better, especially when your subject is moving.
Your 500 is non-IS, right? I'd think as a general rule you'd want to shoot for a minimum shutter of 1/1000 with that lens whenever possible, and again, the more the better. If you have to ramp up the ISO to get there, then ramp it up. With the IS (and of course less focal length) on your 70-200 you can get away with less, but still, more is better, especially if you're like me and get excited at the moment of truth, which leads to poor technique, which in turn leads to wishing for more shutter speed. ;-)
Whoops just re-read your post and now see that your 500 is indeed IS. Maybe that means 1/500 is a more reasonable minimum, maybe not. Once you get up in the range of 400mm+, technique becomes critical.
Anyway, having said all that,
now I'll say go ahead and try it. Go out and shoot with both your cameras in various types of light at every ISO you have available, right up to 3200, and see what you get. Then learn how to make it better. (You're shooting RAW, right?)
And practice, practice, practice. No lions or zebras in your neighborhood? Practice on trucks and motorcycles and bicycles and squirrels. That way, when the once in a lifetime opportunity presents itself, you won't be practicing anymore — you'll be ready.
Nill 
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