I have see quite a few Cobalt systems that run 200 sites on a
450Mhz AMD k6-2 and 256Meg of ram. So unless you are running
a lot of dynamic content proc speed does not matter much. As
far as ram, depends on the content you are serving. Lots of
the same file, ram will help. If every request is different
ram does not help as much.

As for separate servers, you can just use the access file
to send requests from the internal network to one doc root,
while external requests go to another. There is really good
doc for this on the apache web site.