It all depends on your budget. Bluecoat has some of the better ones. Unsure if you mean 200-300 disk cache or you mean 200-300 m/second transfer rates. The disk is no issue the rate make it pretty expensive.
If you would rather spend your time than money you can build a squid proxy running on your favorite linux variant. It can do everything the commercial ones can it just takes good understanding of server design to maximize the performance.