Scaling static content delivery capacity?

I have been running a HTTP mirror site for free software on a single
server for the past 13 months and it has now grown to the stage
where I need to scale up, but I can't actually scale this machine
any further (vertically).

My first instinct was to find another machine of similar specs, get
another RAID array and mirror my system entirely, then round-robin
DNS them. This is affordable but I'm not convinced it is the most
cost-effective solution, nor an efficient use of the resources. It
would also become increasingly difficult to manage with the addition
of each node.

Right now I deliver files directly to the client from direct-attached
RAID storage. My file set is large and growing and I have plenty of
room
to scale there by simply adding more RAID enclosures. What I am yet to
settle on is a method of scaling my delivery capacity as my audience
expands.

I am imagining I would place a cluster of reverse-proxy servers in
front
of my current server, effectively making it the "origin" server.

The proxy servers would each have a fraction of the disk space of my
current server, just enough to handle current downloads. Perhaps a
couple
of small 15KRPM disks in RAID-0.

Each new request should be forwarded to the proxy server bearing the
least load at the time of the request (using something like
mod_backhand
for Apache 1.3). The proxy will then fetch the file from the origin
server and deliver it to the client.

The reason I am holding off on deploying such an architecture is
because
I am not totally convinced by it. Also, mod_backhand/Wackamole is the
only software I have found that would enable this kind of setup, and
I'm
interested in what other methods are being used out there.

I realize that most of big corporations have moved towards CDNs, but
how
would somewhere like CNET have scaled their delivery capacity before
outsourcing it to Kontiki/Akamai?

If you can offer me any advice, feedback on my architecture, or topics
I
should read up on, I would really appreciate it.

Cheers
-colin