Mary,
To answer your question, it varies depending upon multiple factors. Generally the higher you go, the less useful load can be carried.
As a rough guide, light helicopters, (Bell 206L4- with TRACS, Bell 407, B3 Astar), can all do useful work up to around 12,000 to 14,000 feet. Above that and you are pretty much having to start stripping it down.
As for the Mil-17....I do not know.