First, why I ask: Now that reputation points on StackOverflow have real value (you can spend them on damn good answers by offering bounties) I'd like to monitor questions I might be able to answer and pounce on them! (Also the fact that others answer my questions in real time makes StackOverflow amazingly handy, so it would be nice to reciprocate.)
To ask this as a more general question, not specific to StackOverflow:
(Or, for StackOverflow addicts, what's your setup for pouncing on new questions and can you generalize it for monitoring other web content in real time?)
What kind of delay will there typically be in an RSS feed vs polling and scraping?
That depends on what sort of caching the site uses for its RSS feed. On SO there is no noticeable delay on most feeds, but you will need to poll the feed every minute or so. A program can not automagically get notifications from the site itself unless there is some sort of ping service on site in question.