• If you are using a CGI program that takes any kinds of parameters, you will need to cache a copy for each of these.
• How are you gonna determine when a change should occur? By an arbitrarily chosen timespan, or by detecting changes somewhere else?

If you have pages that are generated from say a database or some similar, and doesn't change all that frequently - by manual or timed updates, I'd suggest that you instead produce a new set of HTML pages from your DB (or what it is) upon every change instead. That is a lot more simple approach to reducing load. This assumes that your CGI's don't take any parameters etc, in which case a caching approach is so-so anyways.