Powell's credibility has been damaged by the WMD issue. He made a case to the UN which turned out to be based on bad evidence. It may or may not have been his fault, but he either gave a misleading or hyped case, or he failed to realize its possible inaccuracies.
Cheney is political poison. Bush would be doing himself a favor if he got rid of him, but the action would appear to be a concession to administration critics, a sign of weakness in the campaign, and a chink in the apparent invulnerability of the campaign. Cheney has done nothing to imply that he might leave for health reasons, though the door is by no means closed to that tactic.
There are a lot of names being tossed around as replacements, and they're all noteworthy for how good they look on paper. The simple truth is that pretty much any nationally prominent republican looks better than Cheney at this point.
My prediction is that Cheney will stay on board. Bush won't ask him to leave unless considerable pressure builds for it to happen. The campaign still has Karl Rove and plenty of cash to spend (though they're wasting money on some terrible ads).
Here's an article about the scope of cheney's influence. It's interesting reading, and is rather short:
http://usatoday.com/news/washington/...v-cheney_x.htm