Originally posted by Marinkatomb
If two 2600 players played each other constantly with a 1300 rating and one of them won 100% of 100 games, then the winner would eventually hit a peak rating which he couldn't rise above. If you look at the graph of one of the top rated players on this site and look for a win against a 1300 player, you'll notice that their rating doesn't increase by a si between them, no further increase/decrease would be possible without a change in result...
For the K constant of 24 (about 2100 to 2400 here), you're close - Any rating differential of 597 or greater won't net you any rating increase. But at the lower ratings where the K constant is 32, any rating differential of 720 or more won't get you a rating increase. (Verified with my handy-dandy RHP rating spreadsheet.
The formula for the Delta R at which you cease getting rating points is:
Delta R = 400 * log ((K/(K - 0.5)) -1)
The 0.5 constant in the equation is the rounding down of 1/2 rating point. (Any rating increase less than 0.5 is rounded down to zero, and you get bupkis.)