hi,
i faced a similar question for a client with a lot of files and found out, after a lot of testing
and benchmarking, it nearly doesnt matter if we serve those files from a ram-tmpfs or
use the systems os-cache (given, we have plenty of it). so the os (linux) seems to do a good job.
the servers were designed to serve static files only.
YMMV
regards,
mex