Improving performance by using an additional static file server
Varnish Cache is great for this!
(source: mocsystems.com)
You may find this article interesting.
I'd say the first thing to do is to just start with mod_expires
to reduce static file requests, then perhaps add mod_deflate
to lower bandwidth.
Only once you actually see serious load then start partitioning on to seperate servers.
Moving static requests to a different vserver on the same apache (probably using the same document root) should be a good first step at partitioning.