I have asked this question a little while ago, and I found these answers from the friends here:
check out the "lantronix" website, they provide a broad range of ethernet modules and boards in various models and sizes.
if you found something which provides real 100mbits/sec ethernet, please let me know. I have the same problem here.
I use UDS-100 external converter to control serial LCD over Ethernet. Works just fine. 100M is not attainbale since the RS232 port is way too slow to approach any speed in that range.
However the internal web server is pretty fast, still we are talking few html files as part of the config menu so the speed is really not relevant.