If the only requirement from the light server is for it to serve static objects, you can get away with non-Apache servers, which have an even smaller memory footprint and even better speed. Most of these servers don't have the configurability and flexibility provided by the Apache web server, but if those aren't required, you might consider using one of these alternatives as a server for static objects. To accomplish this, simply replace the Apache web server that was serving the static objects with another server of your choice.
Among the small memory-footprint and fast-speed servers, thttpd is one of the best choices. It runs as a multithreaded single process and consumes about 250K of memory. You can find more information about this server at http://www.acme.com/software/thttpd/. This site also includes a very interesting web server performance comparison chart (http://www.acme.com/software/thttpd/benchmarks.html).
Another good choice is the kHTTPd web server for Linux. kHTTPd is different from other web servers in that it runs from within the Linux kernel as a module (device-driver). kHTTPd handles only static (file-based) web pages; it passes all requests for non-static information to a regular user space web server such as Apache. For more information, see http://www.fenrus.demon.nl/.
Boa is yet another very fast web server, whose primary design goals are speed and security. According to http://www.boa.org/, Boa is capable of handling several thousand hits per second on a 300-MHz Pentium and dozens of hits per second on a lowly 20-MHz 386/SX.