hi;
i think i shall add my 2cents.the network speeds mentioned in above link are using sockets i.e.involving software overheads.the network speed of either 10 or 100 Mbps is not controlled by DS80c400 crystal frequency.it is done an external IC called PHYreceiver.it clocks by a spearate oscillator and uses
2.5Mhz crystal for 10BaseT and 25 Mhz for 100Base-T. This clock basically exhanges 4bit chunks and does shift in/out to/from cable.i believe there shall be a lower speed limit so that 25 Mhz signal is acceptable for On-chip MII on DS80c400.
Pranav