Time-of-day caching?

[Brent]

Quick note of intro: I'm certainly not a highly-experienced sys admin or
developer -- more of a "gifted amateur" as a friend once called me :)
Anyway:

I have a large and fairly complex WordPress-based site that's become a
lot more popular than expected, and is often close to maxing out my
hardware. My challenge in optimizing the site is that all of the most
popular pages/files are very time-sensitive -- they're updated at
specific times of day, and are very heavily downloaded as soon as
they're available. So: I need those pages/files to be flushed from the
cache the moment they change, while everything else is cached more
aggressively. 

I haven't found a good solution for this with WordPress plug-ins such as
SuperCache -- they seem to be too "all-or-nothing", flushing too much or
too little.

I've been tinkering with Nginx as a reverse proxy sitting in front of
Apache. I'm getting very good results so far with regular caching, but I
don't see a way to get that "time-based cache granularity" I need. But
as I say, I'm no expert.

Any thoughts? Thanks.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,179244,179244#msg-179244