Or, if you are a "Cheap Bastard (tm)", you can take a copy of Squid (http://squid.nlanr.net), and an OS which can handle transparent proxying (Solaris, Linux, etc), a high performance RAID, and have yourself the equivilent. Squid is a heirarchial web cache/proxy/accelerator available for free.
i love squid. i use it as a nontransparent proxy and also as the web server accelerator for all of the content we publish. squid is really cool, especially squid-novm. i worry a bit about the supposed proxy capabilities of these other os's. we don't think of ourselves as idiots here when it comes to kernel programming, and so the fact that it's taken a year of wizard level kernel muckraking to get a system that can do thousands of simultaneous transparent sessions makes me think that it's actually a hard problem. we got basic kernel transparency working in an afternoon. the devil is in the details though.