Arduino-like system-on-chip?
Well, an Ardunio can access ethernet and can read from an SD card.

That being said check out the BeagleBoard for a more powerful development board.  The current version doesn't have ethernet on board but works fine with several USB ethernet/wifi adapters, the new beagleboard-xM has ethernet on board.  They are both cortex-A8 based boards, with the xM being a bit faster and having more ram (600mhz vs 1ghz and 256MB vs 512MB).  Theres are also some other differences.  Both run linux among other operating system choices.